GIFT  OF 


OF  THE 

CALIFORNIA  ACADEMY  OF  SCIENCES 


THIRD  SERIES. 
GEOLOGY.  VOL.  I,  No.  i 


THE  GEOLOGY  OF  SANTA  CATALINA 

ISLAND. 


BY 

WILLIAM  SIDNEY  TANGIER  SMITH, 

Candidate  Ph.  D.,  University  of  California. 


WITH  THREE  PLATES. 


Issued  February  4,  1897. 


SAN  FRANCISCO: 

PUBLISHED  BY  THE  ACADEMY. 

1897. 


PUBLICATION  COMMITTEE. 

WILLIAM  E.  RITTER,  Chairman. 
DAVID  S.  JORDAN,  G.  P.  RIXFORD. 


EDITORS  OF  GEOLOGICAL  PUBLICATIONS. 
JAMES  PERRIN  SMITH,  ANDREW  C.  LAWSON. 


ERRATA. 

P.  42,  line  ii  from  the  bottom,  for  "of,"  read  "on." 

P.  53,  line  9,  for  "Plate  III,  fig.  i,"  read  "Plate  II,  fig.  i." 

P.  69,  line  6  from  the  bottom,  for  "Plate  III,  fig.  i,"  read 

"Plate  III,  fig.  2." 

P.  71,  line  10,  for  "steam,"  read  "stream." 
P.  54,  line  10.  The  statement  that  certain  areas  appear  on 
the  map  without  definite  boundaries  is  not  in  accord- 
ance with  the  fact.  Owing  to  a  misunderstanding  on 
the  part  of  the  lithographer,  the  areas  in  question  are 
delimited  by  the  usual  dotted  line. 


PROCEEDINGS 


CALIFORNIA  ACADEMY  OF  SCIENCES 


THIRD  SERIES. 


GEOLOGY. 


VOL.  I,  No.   i 


THE  GEOLOGY  OF  SANTA  CATALINA 

ISLAND. 


BY 

WILLIAM  SIDNEY  TANGIER  SMITH, 

Candidate  Ph.  D.,   University  of  California. 


WITH  THREE  PLATES. 


Issued  February  4,  1897. 


SAN   FRANCISCO: 

PUBLISHED  BY  THE  ACADEMY. 

1897. 


THE  GEOLOGY  OF   SANTA  CATALINA  ISLAND. 

BY    WILLIAM    SIDNEY    TANGIER     SMITH. 

Candidate  Ph.  Z>.,  University  of  California. 

CONTENTS. 

PAGE. 

PLATES  I— III. 

I.     INTRODUCTION 2 

1.  LITERATURE 2 

2.  GENERAL  DESCRIPTION 3 

II.     TOPOGRAPHY 5 

1.  MAJOR  FEATURES 4 

Main  Ridge 4 

Types  of  Topography 5 

Slope  of  Summits 6 

Two  Types  of  Drainage 7 

2.  MINOR  FEATURES 8 

Echo  Lake 8 

Sea  Cliffs 9 

Bays 10 

Beaches 10 

Terraces n 

III.    GEOLOGY. 13 

A.     ERUPTIVE  ROCKS 14 

1.  DIORITE 14 

Macroscopic  Characters 14 

Microscopic  Characters 15 

2.  PORPHYRITE 19 

Occurrence 19 

Character 19 

Macroscopic  Characters 20 

Microscopic  Characters 20 

Analysis 25 

Inliers  of  Basement  Rocks 25 

Porphyrite  Dykes 26 

3.  RHYOLITE 28 

Occtirrence 28 

Macroscopic  Characters 28 

Microscopic  Characters , 29 

4.  ANDESITE 30 

Occurrence 30 

Macroscopic  Characters 31 

Microscopic  Characters 32 

Glassy  Fades  35 

Basaltic  Fades 37 

Analysis 41 

[  i  |  January  12,  1897. 


244561 


2  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

PAGE. 

5.     RELATIVE  AGE 41 

B.  TUFF  AND  DIATOMACEOUS  EARTH 42 

1.  OCCURRENCE 42 

2.  TUFF 43 

3.  SHALE 43 

Microscopic  Characters 44 

Character  of  the  Organic  Remains 45 

Chemical  Characters 48 

Origin  of  the  Shale 49 

Analysis  of  Limestone 50 

C.  SEDIMENTARY  DEPOSITS 51 

D.  BRECCIA 52 

E.  BASEMENT  SERIES 54 

1.  QUARTZITE 54 

Macroscopic  Characters 55 

Microscopic  Characters 57 

2.  ACTINOLITE  AND   HORNBLENDE  SCHISTS 58 

3.  SERPENTINE 58 

4.  TALC-SCHIST 60 

5.  ORIGIN  OF  THE  SERPENTINES 61 

6.  GARNET- AMPHIBOLITE 62 

IV.    GEOMORPHOGENY 65 

1.  SUBMARINE  TOPOGRAPHY 65 

2.  OUTLINE  OF  HISTORY 67 


I.    INTRODUCTION. 

i .     LITERATURE. 

THE  existing  literature  bearing  upon  the  geology  of  Santa 
Catalina  is  very  limited,  consisting  of  a  short  note  in  Whit- 
ney's Geology,1  a  brief  report2  and  other  scattered  notes  in 
the  various  Annual  Reports  of  the  State  Mining  Bureau, 
and  a  recent  account  of  the  topography  of  the  island,  by 
Prof.  Lawson.3  The  report  of  the  State  Mining  Bureau 
is  not  only  superficial,  but  very  inaccurate.  In  Whitney's 
report  the  absence  of  terraces  is  noted,  contrasting  with  the 
neighboring  islands,  and  the  suggestion  is  made  that  this 
island  may  be  sinking.  Both  of  these  points  are  elaborated 

1Geol.  Surv.  of  Cal.,  Geol.,  Vol.  I,  pp.  182-186. 
2 Tenth  An.  Rept.,  State  Mineralogist,  pp.  277-281. 

8 "The  Post-Pliocene  Diastrophism  of  the  Coast  of  Southern  California,"  by  Andrew 
C.  Lawson.  Bull.  Dept.  Geol.,  Univ.  Cal.,  Vol.  I,  No.  4,  pp.  135-139' 


GEOL.— VOL.  I.]        SMITH— SANTA   CATAL1NA  ISLAND.  3 

by  Prof.  Lawson,  who  further  calls  attention  to  the  older 
topography  of  this  island. 

2.     GENERAL    DESCRIPTION. 

Santa  Catalina  Island,  one  of  the  group  known  as  the 
Channel  Islands,  off  the  coast  of  southern  California,  lies 
about  20  miles  south  of  San  Pedro  Hill,  the  nearest  point  on 
the  mainland.  At  about  the  same  distance  south  of  Santa 
Catalina  lies  the  island  of  San  Clemente,  the  three  eleva- 
tions being  nearly  in  a  straight  line. 

The  general  trend  of  the  island  is  northwest  by  west. 
Its  length  is  approximately  twenty-one  miles,  with  an  aver- 
age width  of  three  miles,  varying  from  half  a  mile  at  the 
isthmus  to  about  eight  miles  in  the  widest  part.  The  pre- 
vailing winds  are  from  west  to  southwest,  and  the  waves 
exert  their  greatest  force  on  the  southwest  face  of  the  island. 
They  are,  however,  by  no  means  inactive  on  the  landward 
side,  as  is  shown  by  the  rapidly  retreating  shore-line. 

The  only  settlements  on  the  island  are  the  summer  resort 
at  Avalon,  and  a  small  community  at  the  isthmus.  Besides 
these,  a  few  solitary  houses  are  located  at  different  points  on 
the  coast.  The  island  was  once  occupied  by  Indians,  and  evi- 
dences of  their  camps  occur  frequently  in  the  form  of  shell 
fragments,  rounded  stone  implements,  and  earth  blackened 
by  the  camp  fires.  Owing  to  its  ruggedness  and  the  scarcity 
of  water,  the  island  is  habitable  in  only  a  few  places.  There 
are  half  a  dozen  or  more  springs  and  creeks  which  do  not 
dry  up  during  the  summer,  and  a  few  wells  supply  the  other 
points.  All  the  water  is  decidedly  alkaline. 

The  vegetation  consists  chiefly  of  herbage  and  shrub- 
bery or  underbrush,  cactus  forming  an  important  part.  The 
larger  trees,  except  for  a  few  dwarf  oaks,  are  confined  to 
the  bottoms  of  the  canons.  The  summits,  in  general,  are 
bare  of  everything  except  grass  and  cactus,  but  the  majority 
of  the  slopes  are  thickly  covered  with  an  often  impenetrable 
growth  of  scrub-oak,  greasewood  (Adenostoma  fascicula- 
tumf),  and  elder,  intermingled  with  cactus.  It  is  note- 


4  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

worthy  that,  in  spite  of  the  oftentimes  luxuriant  vegetation, 
the  soil-covering  is  generally  very  thin,  and  the  underlying 
formations  are  constantly  exposed. 


II.    TOPOGRAPHY. 

i.     MAJOR  FEATURES. 

Main  Ridge. — The  island  is  traversed  from  end  to  end  by 
a  single  main  ridge,  with  branch  ridges  running  out  on  either 


FIGURE  i — The  Isthmus,  looking  south. 

side.  Beginning  about  a  mile  from  the  southeastern  extrem- 
ity of  the  island,  this  ridge  makes  a  bold  sweep  around  the 
head  of  Avalon  Canon  to  a  point  nearly  west  of  Avalon. 
There  it  makes  an  abrupt  turn,  almost  at  right  angles,  and 
then  follows  very  nearly  the  line  of  the  northern  coast,  at  an 
average  distance  of  about  a  mile  from  the  shore,  till  it  reaches 
the  isthmus.  (See  fig.  i.)  This  is  alow  divide,  in  the  form 
of  a  saddle,  with  very  gentle  slopes.  It  has  a  length, 
between  the  bounding  hills,  of  less  than  a  quarter  of  a  mile, 


GEOL.— VOL.  I.]        SMITH— SANTA  CATALINA  ISLAND.  5 

and  its  greatest  elevation  is  about  twenty  feet.  At  either 
end  of  the  isthmus  the  hills  rise  very  abruptly  to  the  main 
ridge,  which  is  here  from  800  to  900  feet  in  height.  West 
of  this  point  the  ridge  has  two  divisions,  which  unite,  less 
than  a  mile  and  a  half  beyond,  to  form  again  a  single  ridge, 
continuing  to  the  end  of  the  island.  On  this  end  the  ridge 
lies  nearer  the  south  than  the  north  shore.  One  noticeable 
feature  of  this  main  watershed  is  the  general  uniformity  of 
its  height.  For  the  greater  part  of  its  length  the  variations 
in  altitude  are  not  more  than  two  or  three  hundred  feet,  the 
average  elevation  being  about  1,400  feet.  The  two  greatest 
elevations  are  near  the  center  of  the  island,  the  peak  known 
as  "  Orizaba"  (or  "  Brush  Mountain  "),  marked  2,109  feet 
on  the  map,  and  "  Black  Jack,"  the  peak  about  a  mile  to 
the  northeast  of  this,  about  a  hundred  feet  lower. 

Types  of  Topography. — The  general  character  of  the 
topography  is  very  bold  and  rugged,  and  shows  an  advanced 
stage  of  development.  A  general  view  of  the  island  from 
almost  any  point  gives  an  impression  of  a  close  succession 
of  sharp,  steep  ridges  and  V-shaped  canons.  One  of  the 
most  marked  examples  of  this  effect  is  in  the  slopes  of 
Avalon  Canon,  particularly  on  the  west  side,  when  seen 
from  the  opposite  summits. 

Viewed  in  detail,  the  island  shows  two  prevailing  forms 
of  topographic  relief:  (i)  the  sharp  ridges  and  V-shaped 
canons  just  referred  to,  and  (2)  the  rounded  and  level  forms 
belonging  to  an  older  topography.  The  slope  of  the  canon 
walls,  in  the  first  type,  is  usually  steep,  occasionally  having 
an  angle  of  40°  or  over.  The  first  form  is  the  prevailing 
one  in  most  parts  of  the  island,  masking  the  remnants  of  the 
second. 

The  second  type  of  topography  is  strongly  contrasted 
with  the  first.  It  is  found  in  the  higher  parts  of  the  island, 
best  developed  in  the  eastern  end.  It  is  shown  in  the  level 
character  of  the  main  ridge,  and  of  several  of  the  minor 
ridges  which  approximate  it  in  altitude.  These  latter  are 
(i)  the  principal  ridges  between  Middle  Ranch  Canon  and 


6  CALIFORNIA  ACADEMY  OF  SCIENCES.         [PROC.  30  SER. 

the  main  ridge  bounding  Avalon  Canon  on  the  west;  (2) 
the  ridge  connecting  the  main  ridge  with  the  point  north  of 
Whitley's  Cove;  and  (3)  a  portion  of  each  of  the  ridges 
running  from  the  main  ridge  into  the  Little  Harbor  region 
(which  comprises  the  semicircular  area  within  a  general  ra- 
dius of  about  three  miles  from  Little  Harbor). 

In  the  lower  portions  of  the  Little  Harbor  region  this  sec- 
ond type  of  topography  again  appears.  Within  this  area 
the  tributary  ridges,  radiating  from  a  central  point  not  far 
from  Little  Harbor,  rise  to  the  higher  slopes  by  a  long, 
moderate  incline.  Beginning  at  the  shore-line,  with  a  cliff 
of  from  200  to  300  feet,  the  rise  above  this  is  very  gradual,  till, 
at  an  average  distance  of  a  mile  and  a  half  from  the  water,  a 
height  of  about  600  or  700  feet  is  reached.  Beyond  this 
the  grade  increases,  and  an  altitude  equal  to  that  of  the  main 
ridge  is  soon  reached,  usually  some  little  distance  from  the 
main  ridge  itself.  Standing  on  the  lower  and  more  level 
portion  of  this  area,  and  looking  either  toward  the  isthmus 
or  in  the  opposite  direction,  one  sees  a  great  amphitheater, 
the  distant  ridges  rising  one  above  another,  like  gigantic 
tiers  of  seats,  up  to  the  main  ridge.  Were  it  not  for  the 
recent  stream  erosion  we  should  thus  have  in  the  immediate 
neighborhood  of  Little  Harbor  an  almost  even  surface,  with 
a  gentle  seaward  slope.  The  present  drainage,  however, 
has  dissected  this  surface,  cutting  channels  some  of  which, 
in  their  lower  stretches,  have  a  width  of  100  yards  or  more, 
and  a  depth  of  perhaps  200  feet.  In  places  the  streams 
have  made  considerable  deposits,  and  at  a  number  of  points 
these  have  been  cut  through,  in  very  recent  times,  to  a  max- 
imum depth  of  about  twenty-five  feet,  at  some  distance  from 
the  shore. 

Slope  of  Summits. — It  has  been  seen  that  the  main  ridge 
and  certain  of  the  branch  ridges  are,  in  a  general  way,  level 
in  the  direction  of  the  length  of  the  island.  In  those  por- 
tions of  the  main  ridge  on  either  side  of  Avalon  Canon 
which  are  oblique  to  the  trend  of  the  island,  the  generally 
level  summits  are  seen  to  slope  at  an  angle  of  a  little  over 


GEOL.-VOL.  I.]        SMITH— SANTA  CATALINA  ISLAND. 


one  degree  toward  the  northern  shore.  (See  figs.  2  and 
3.)  In  fig.  2,  Black  Jack  and  Orizaba  and  a  portion  of 
the  ridge  between  Silver  and  Middle  RancrT  Canons  are 
seen  above  the  main  ridge.  At  the  "  west  end  "  (that  por- 


South. 


North. 


FIGURE  2— Outline  of  the  summit  of  the  main  ridge  west  of  Avalon  Canon, 
as  seen  from  the  summit  of  the  ridge  on  the  opposite  side  of  the  canon. 

tion  of  the  island  west  of  the  isthmus),  the  more  northerly  of 
the  two  branches  of  the  main  ridge  has  an  average  height 
throughout  its  length,  about  200  feet  lower  than  the  other. 
Thus  it  appears  that  in  transverse  section  the  island  shows 
a  general  slope  toward  the  mainland. 


East. 


West. 


FIGURE  3— Outline  of  the  summit  of  the  main  ridge  south  of  Avalon  Canon, 
as  seen  from  the  summit  of  the  ridge  on  the  opposite  side  of  the  canon. 

Two  Types  of  Drainage. — The  principal  stream  canons 
running  from  the  main  watershed  are  of  two  types,  which 
may  be  readily  distinguished  on  the  map.  Those  canons 
which  have  their  mouths  on  the  northern  or  landward  coast 


8  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

are  broad  and  open  stream  valleys,  while  those  running 
down  to  the  opposite  shore  have  a  long  and  trough-like 
character  which  they  preserve  to  the  shore-line.  These 
two  types  are  quite  pronounced  over  all  the  southeastern 
division  of  the  island,  while  at  the  isthmus  the  two  harbors 
—  which  are  merely  submerged  stream  valleys  —  still  show 
the  same  contrast.  In  the  western  end  this  characteristic, 
though  still  evident,  is  not  so  marked.  The  narrow,  trough- 
like  canons  are  occasionally  somewhat  broader  in  their  upper 
portions  than  near  their  mouths,  where  they  are  frequently 
mere  rocky  gorges.  The  most  pronounced  example  of  the 
narrow  type  is  Silver  Canon,  whose  walls  near  the  mouth 
rise  to  a  height  of  over  1,000  feet,  while  the  distance  between 
them  at  the  base  is  in  places  not  more  than  from  twenty-five 
to  a  hundred  feet.  The  length  of  this  canon  is  about  three 
miles.  Avalon  Canon,  a  good  example  of  the  other  type, 
has  a  length  of  about  two  miles,  with  a  mean  width,  from 
watershed  to  watershed,  of  somewhat  more  than  that. 
From  the  main  ridge  on  either  side  a  great  number  of 
rather  short  and  steep  V-shaped  canons  are  tributary  to 
the  main  valley,  these  stream  beds  making  the  descent  of 
1,200  or  1,400  feet  within  an  average  distance  of  about  a 
mile. 

All  the  forms  of  topography  thus  far  described  are  largely 
independent  of  the  material  from  which  they  are  carved; 
that  is,  variation  in  the  character  of  the  rocks  has  but  little 
connection  with  variation  in  topographic  form. 

2.     MINOR  FEATURES. 

Echo  Lake. — There  is  one  small  lake  on  the  island,  sit- 
uated about  a  mile  to  the  northeast  of  Black  Jack,  at  an 
altitude  of  about  1,300  feet.  This  belongs  to  the  class  of 
ephemeral  lakes.  Visiting  it  two  summers  in  succession,  at 
the  same  season,  the  writer  found  it,  the  first  time,  a  shal- 
low pond  about  100  yards  long,  while  the  next  year  it  was 
entirely  dry.  It  is  a  small  drainage  lake,  without  outlet, 
probably  shut  in  by  faulting. 


GEOL.— VOL.  L]        SMITH— SANTA  CATALINA  ISLAND.  9 

Sea  Cliffs. — Except  for  the  openings  formed  by  the  canon 
mouths,  cliffs  surround  the  island  on  all  sides,  running  from 
one  or  two  hundred  feet  to  1,400  feet  or  more  in  height. 
The  boldest  and  highest  cliffs  are  found  at  the  west  end,  and 
between  Silver  Canon  and  the  southeastern  extremity  of  the 
island.  The  highest  of  all  are  just  to  the  east  of  Silver 
Canon,  where  the  waves  have  cut  across  the  end  of  a  minor 
ridge  whose  altitude  equals  that  of  the  main  ridge.  These 
cliffs,  although  furnishing  excellent  geological  sections,  are 
wholly  inaccessible  at  nearly  all  points,  owing  to  their  height, 
the  angle  at  which  they  meet  the  water,  and  the  absence  of 
beaches. 

The  cliffs  are  rapidly  receding,  in  many  cases  more  rapidly 
than  the  streams  which  trench  their  surfaces  can  cut  down 
their  channels.  This  is  shown  by  the  V-shaped  openings 
on  the  face  of  the  cliff,  from  50  to  200  feet  or  more  above 
the  water.  Such  are  the  mouths  of  the  canons  draining  the 
southern  slopes  of  the  main  ridge  at  the  head  of  Avalon 
Canon.  These  open  on  the  southern  coast,  about  two  miles 
to  the  east  of  the  entrance  to  Silver  Canon.  The  rapidity 
of  the  cliff-cutting  here  will  appear  the  more  remarkable 
when  it  is  known  that  these  streams,  though  draining  com- 
paratively small  areas,  are  torrential  in  character.  It  must, 
however,  be  remembered  that  they  are  active  only  during 
the  rainy  season.1 

In  addition  to  these  larger  V-shaped  openings,  several 
smaller  ones  were  seen  along  the  higher  parts  of  the  cliff, 
less  than  a  mile  to  the  east  of  the  entrance  to  Silver  Canon. 
These,  from  the  water  side,  present  the  appearance  of  a 
stream  draining  outward  over  the  face  of  the  cliff.  From 
above,  however,  it  is  seen  that  the  drainage  is  inland,  toward 
Silver  Canon.  This  phenomenon  is  due  to  the  cutting  back 

*A  phenomenon  similar  to  that  above  described  has  been  observed  by  the  writer  at 
several  points  along  the  California  coast  between  Port  Harford  and  Santa  Monica.  Here 
the  recent  streams  have  carved  narrow  channels  in  the  surface  of  the  lowest  terraces 
which  border  the  shore,  and  have  formed  clear-cut  V's  on  their  upper  edge.  The  cause 
here  (unlike  that  in  the  case  of  Santa  Catalina)  is,  no  doubt,  that  an  insufficient  time 
has  elapsed,  since  the  elevation  of  the  coast,  for  the  streams  to  deepen  their  channels 
further. 


10  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

of  the  watershed  so  rapidly  that  the   drainage  has  not  had 
time  to  adjust  itself  to  the  changed  conditions. 

Bays. — The  coast  of  the  island,  particularly  on  the  land- 
ward side,  is  indented  with  numerous  bays.  On  the  north  side, 
partly  on  account  of  less  active  cutting  along  the  coast,  and 
partly  on  account  of  the  more  open  canons  whose  submerg- 
ence has  produced  the  bays,  they  are  wider,  and  generally 
furnish  safe  landing  places.  On  the  other  side  of  the  island, 
although  there  are  numerous  recesses  in  the  shore-line  (par- 
ticularly of  the  west  end),  these  openings  are  generally 
surrounded  by  high  cliffs,  and  there  are  only  two  bays, 
Catalina  Harbor  at  the  isthmus,  and  Little  Harbor. 

Beaches. — Several  canons  on  the  south  side  of  the  island, 
while  not  forming  bays,  have  beaches  at  their  mouths.  In 
many  cases,  both  here  and  on  the  northern  coast,  the  beaches 
have  been  built  up  by  wave  action  so  as  to  form  along  the 
shore  a  barrier  from  five  to  ten  feet  higher  than  the  area  just 
behind.  The  beaches,  in  general,  consist  of  coarse,  well 
rounded,  and  flattened  shingle,  though  one  or  two  excep- 
tions were  seen  where  the  beach  was  largely  composed  of  a 
rather  fine  sand.  Apart  from  the  beaches  which  mark  the 
entrance  to  the  larger  canons,  there  are  a  few  very  narrow 
beaches  for  short  stretches  at  the  base  of  the  cliffs,  only 
on  the  landward  side.  These  are  in  general  accessible  only 
at  low  water. 

The  beaches  as  a  rule  are  curved  in  outline,  concave  to- 
ward the  ocean.  A  marked  exception  to  this  is  seen  in  the 
projecting,  tongue-like,  Pebbly  Beach.  This  has  been  built 
up  by  the  opposing  action  of  two  series  of  waves,  which, 
coming  from  either  direction  along  the  coast,  meet  at  this 
point.  Not  only  does  the  beach  exhibit  the  barrier-like 
character  mentioned  above,  but  its  outer  surface  shows  a 
series  of  narrow  terraces  formed  by  the  waves.  As  many 
as  six  were  seen  at  one  point. 

Another  form  is  shown  in  the  hook  which  marks  the  en- 
trance to  Catalina  Harbor,  and  is  known  as  Ballast  Point. 
This  is  built  of  coarse  shingle,  some  of  the  material  compos- 


GEOL.— VOL.  I.]         SMITH— SANTA  C ATA  LIN  A  ISLAND.  II 

ing  it  having  a  diameter  of  about  a  foot.  High  winds  blow 
daily  through  the  narrow  pass  at  the  isthmus,  causing  a  strong 
inward  current,  which  is  gradually  bringing  about  a  shoaling 
of  the  harbor.  Thus  here,  as  at  Pebbly  Beach,  the  accu- 
mulation of  shore-drift,  through  the  action  of  waves  and 
currents,  has  more  than  kept  pace  with  the  sinking  of  the 
island. 

Terraces. — The  pronounced  contrast  which  Santa  Cata- 
lina  presents  in  its  topography,  not  only  to  the  adjacent  land 
areas,  but  to  the  greater  part  of  the  coast  of  California,  has 
already  been  shown  by  Prof.  Lawson.1  The  most  striking 
difference  is  in  the  marked  absence,  on  this  island,  of  the 
terraces  which  are  so  clear-cut  and  pronounced  on  the  slopes 
of  San  Pedro  Hill  and  San  Clemente.  With  but  two  excep- 
tions, Santa  Catalina  is  devoid  of  any  evident  terracing  from 
one  end  to  the  other.  The  terrace-like  character  of  the  lower 
levels  of  the  Little  Harbor  region  (already  described)  forms 
one  of  these  exceptions.  That  this  is,  in  part,  at  least,  of 
the  nature  of  a  true  terrace,  is  shown  by  the  nearly  level 
character  of  the  various  ridges  in  their  lower  parts,  their 
gentle  seaward  slope,  the  change  in  grade  at  the  rear,  at  an 
altitude  of  600  or  700  feet,  the  planing  off  of  the  upturned 
beds  of  the  basement  series,  with  rolled  pebbles  scattered 
over  the  lower  slopes  of  the  andesite,  besides  more  or  less 
sandstone  and  conglomerate  on  these  slopes  bordering  Mid- 
dle Ranch  Canon.  All  these  point  to  a  time  when  this  re- 
gion contained  a  bay,  into  an  arm  of  which  a  stream,  doubt- 
less an  older  form  of  that  which  now  drains  Middle  Ranch 
Canon,  brought  the  deposits  just  mentioned  (shown  on  the 
map).  It  is  possible  that  there  is,  besides  this,  a  series  of 
such  terraces  within  this  area.  If  so  they  are  not  strongly 
marked,  and  the  fact  could  only  be  established  by  a  more 
detailed  observation  than  the  writer  had  time  for.  Ter- 
racing similar  to  that  found  here  must  at  one  time  have  ex- 

1 "  The  Post-Pliocene  Diastrophism  of  the  Coast  of  Southern  California,"  by  Andrew 
C.  I,awson.  Bull.  Dept.  Geol.,  Univ.  Cal.,  Vol.  I,  No.  4,  pp.  135-139. 


12 


CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 


tended  along  the  cliffs  bordering  the  island,1  but  it  has  been 
since  removed  by  a  prolonged  period  of  active  cliff  erosion. 
That  the  evidence  is  preserved  here  is  due  to  the  fact  that 
the  terracing  extended  so  far  inland.  This  belongs  to  an 
earlier  period  than  the  terraces  of  the  main  coast  and  of  San 
Clemente. 

The  other  terraced  structure  occurs  in  the  canon  back  of 


FIGURE  4 — Dissected  alluvial  fan,  southeast  side  of  Avalon  Canon. 

Avalon  (see  fig.  4) ,  about  half  a  mile  from  the  shore-line,  and 
is  seen  on  both  sides  of  the  canon — which  is  about  a  quarter 
of  a  mile  wide  at  this  point — as  a  broad  platform  extending 
up  some  distance  into  the  canon-like  opening  on  either  side 
of  the  main  valley.  Its  front  edge  has  a  gentle  seaward 
slope,  while  from  front  to  rear  it  rises  gradually  toward  the 
hills.  A  sharp  ascent  of  about' forty  or  fifty  feet  marks  the 

1  Rolled  pebbles  were  found  scattered  over  a  small  area  near  the  southeastern  end 
of  the  island,  at  an  altitude  of  1,000  feet;  also  on  the  main  ridge  south  of  Avalon  Canon, 
at  about  1,400  feet;  but  the  remains  of  an  Indian  camp  within  a  hundred  feet,  in  each 
case,  made  the  evidence  doubtful.  In  neither  case,  however,  were  any  pebbles  found 
among  the  remains  marking  the  camp. 


GEOL.— VOL.  I.]        SMITH— SANTA  CATALINA  ISLAND.  13 

front  of  the  platforms  on  either  side  of  the  canon.  Streams 
have  cut  into  the  surface  and  along  the  sides,  forming  sev- 
eral comparatively  broad  watercourses.  Some  of  these 
streams  have  not  yet  reached  the  level  of  the  main  canon 
where  they  debouch  upon  it,  and  have  formed  small,  rather 
low  and  broad  alluvial  fans  beyond  their  mouths. 

The  material  composing  the  platforms  consists  of  both 
rounded  and  angular  fragments.  From  the  form  of  the 
structures  and  the  form  and  arrangement  of  their  material, 
it  is  evident  that  we  have  here,  not  a  stream  terrace,  as  its 
appearance  might  at  first  indicate,  but  undoubted  alluvial 
fans.1  These  have  been  dissected  and  cut  away  in  recent 
geological  times,  owing  to  the  drowning  of  the  stream  valley 
at  Avalon,  with  a  consequent  shortening  of  the  stream 
courses,  and  a  deepening  of  the  channels. 

III.     GEOLOGY. 

The  basement  series  of  Santa  Catalina  consists  of  crys- 
talline metamorphic  rocks,  principally  quartzite.  This  series, 
with  the  hornblendic  rocks,  the  talc-schist  and  the  serpentine, 
covers  in  a  general  way  the  whole  western  half  of  the  island. 
Besides  the  main  occurrence,  there  are  patches  of  these 
rocks  along  the  main  ridge  to  the  west  of  Avalon  Canon. 
The  basement  rocks  are  cut  by  occasional  dikes,  which  are 
principally  at  the  west  end,  and  have  a  general  northeasterly 
trend. 

The  eastern  end  of  the  island  is  occupied  by  diorite  and 
porphyrite.  Bordering  this  area  on  the  north,  and  of  later 
age,  is  an  area  consisting  of  numerous  flows  of  andesite,  of 
which  there  are  several  other  smaller  occurrences  besides. 
A  particularly  interesting  area  of  these  rocks  is  found  to  the 
east  of  Isthmus  Cove,  where,  interbedded  with  the  volcanics, 
is  seen  a  band  of  tuff  and  diatomaceous  earth. 

The  lower  slopes  of  the   andesites   in   the   Little    Harbor 

Alluvial  fans  are  by  no  means  uncommon  on  the  island,  but  no  other  case  presents 
any  similar  terracing. 


14  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

region  are  covered  with  rolled  pebbles,  with  several  patches 
of  sedimentary  deposits.  In  the  eastern  portion  of  the 
Little  Harbor  region  is  a  small  area  of  rhyolite,  which  was 
not  found  elsewhere.  A  narrow  strip  of  quartzite  breccia 
occurs  at  the  southeastern  extremity  of  the  island. 


A.     ERUPTIVE  ROCKS. 
I.       DlORITE. 

There  are  three  observed  occurrences  of  the  diorite :  one 
along  the  cliff  bordering  the  shore  just  to  the  north  of 
Avalon,  the  second  near  the  head  of  the  canon  back  of 
Pebbly  Beach,  and  the  third  in  the  lower  portion  of  Silver 
Canon.  These  are  apparently  dikes  of  considerable  width. 

Macroscopic  Characters. — The  diorite  is  coarse-grained, 
and  of  a  light  grayish  color,  more  or  less  mottled.  The 
specific  gravity  of  a  specimen  from  Silver  Canon  was  found 
to  be  2.777.  In  the  coarser-grained  specimens  the  separate 
minerals  may  easily  be  seen  without  recourse  to  a  lens. 
With  the  lens  the  rock  is  seen  to  be  composed  of  a  ferro- 
magnesian  mineral,  feldspar  and  a  varying  amount  of 
quartz. 

The  feldspars  range  from  a  somewhat  glassy  condition  to 
one  in  which  they  are  whitish  and  more  or  less  opaque. 
They  constitute,  in  general,  the  principal  mineral  of  the 
diorite.  The  ferromagnesian  mineral  is  hornblende,  dark 
green  in  color,  and  altered  in  part  to  chlorite.  This  mineral 
varies  in  amount  from  a  little  less  than  one-half  to  perhaps 
one-fifth  of  the  surface  area.  Besides  the  hornblende  an 
occasional  leaflet  of  biotite  was  seen  in  the  specimens  from 
near  Avalon,  and  also  in  one  or  two  from  Silver  Canon. 
The  quartz  usually  occurs  in  small  areas  scattered  through- 
out the  mass  of  the  rock.  In  addition  to  these,  magnetite 
is  apparent  in  nearly  all  the  hand-specimens  of  the  coarser- 
grained  varieties,  being  rather  conspicuous  in  one  specimen 
from  the  canon  back  of  Pebbly  Beach. 


GEOL.— VOL.  I.]     SMITH— SANTA  CATALINA  ISLAND.  15 

Microscopic  Characters. — Under  the  microscope  the  dio- 
rites  are  found  to  have  a  nearly  even-grained^  hplocrys- 
talline  structure,  and  to  be  composed  essentially  of  a  lime- 
soda-feldspar,  hornblende  and  occasional  biotite,  with  free 
quartz  always  present  in  varying  amounts.  Augite  is  also 
present  in  some  of  the  slides,  and  in  nearly  all  magnetite 
is  an  important  constituent.  No  apatite  was  observed  in 
any  of  the  sections.  There  is  an  occasional  tendency  to 
a  porphyritic  development  among  the  feldspars.  Mineral- 
ogically  considered,  the  rock  is  a  quartz-hornblende-diorite, 
with  a  tendency  to  lath-shaped  forms  among  the  feldspars. 

The  feldspars  are  in  general  allotriomorphic,  and  tend  to 
develop  crystal  faces  only  occasionally,  where  they  come  in 
contact  with  quartz.  In  some  of  the  slides  many  of  the 
feldspars  are  fairly  clear  and  free  from  inclusions  or  de- 
composition products.  Aside  from  these  the  majority  are 
clouded  by  alteration  products,  which,  in  some  cases,  have 
partly  or  wholly  obliterated  the  traces  of  the  twinning  lamel- 
lag,  which  are  clearly  shown  in  the  fresher  material.  This 
cloudiness  is  apparently  due  in  part  to  a  kaolinization  of  the 
plagioclase,  but  also  to  calcite,  which  occurs  in  small, 
irregular  patches  and  threads  in  many  of  the  sections.  This 
product  is  also  found,  in  some  instances,  in  lines  along  the 
twinning  planes.  Twinning  takes  place  according  to  the 
Carlsbad,  albite  and  pericline  laws.  Pericline  twinning  is 
the  least  frequent,  and  is  usually  seen  under  crossed  nicols 
as  a  series  of  very  fine  lines. 

Excellent  zonal  structure  is  occasionally  seen,  but  is  in- 
frequent. The  varying  optical  orientation  in  such  cases 
shows  that  the  mineral  grows  more  acid  from  the  center 
outward.  Inclusions  in  the  feldspars  are  not  common,  but 
rarely  one  of  the  largest  crystals  contains  from  one  to  a 
number  of  smaller  feldspars  which  are  without  definite  orien- 
tation toward  their  host,  and  without  good  crystal  boun- 
daries. Inclusions  of  small  ragged  flakes  of  hornblende 
or  chloritic  material  are  of  much  more  frequent  occurrence. 
Several  feldspars  occur  packed  with  small,  irregularly 
bounded  sections  of  what  appears  to  be  primary  hornblende. 


1 6  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

The  structure  in  this  case  is  micropoikilitic.  Occasionally 
sections  contain  numerous  brightly  polarizing,  microscopic 
needles,  doubtless  of  hornblende. 

Besides  the  decomposition  products  already  mentioned, 
more  or  less  epidote  is  usually  present,  generally  in  small 
irregular  patches. 

All  the  diorites  have  doubtless  been  subject  to  stresses 
since  they  were  consolidated,  but  only  the  rocks  from  Silver 
Canon  give  any  marked  microscopic  evidence  of  it.  In 
these  rocks  the  feldspars  seem  to  have  been  particularly 
affected,  the  hornblende  and  quartz  showing  little  or  no  evi- 
dence of  strain.  The  evidence  here  is  of  three  kinds — 
altered  optical  properties,  bent  crystals  and  fractures,  which 
may  or  may  not  cause  displacements.  The  extinction  of 
the  feldspars  is  frequently  very  indefinite  and  variable. 
Comparatively  few  of  them  show  good  extinction,  and  even 
these  are  sometimes  considerably  affected  by  cracks.  In 
the  others  strain  shadows  take  the  place  of  the  normal 
extinction.  Bent  crystals  are  not  common,  but  a  few  of 
the  sections  show  a  distinct  curvature,  and  a  corresponding 
alteration  in  extinction.  The  fractures  referred  to  are  not 
the  cracks  frequently  found  in  single  individuals,  but  more 
extensive  ones  passing  from  crystal  to  crystal,  simply  as 
cracks,  or  forming  veins  which  have  been  filled  with  sec- 
ondary matter.  A  few  veins  were  found,  the  most  pro- 
nounced one  extending  irregularly  half  through  a  slide, 
some  of  the  feldspars  on  either  side  having  suffered  slight 
displacement.  This  vein  is  .05  mm.  in  width,  and  is 
filled  principally  with  calcite,  with  more  or  less  chlorite  and 
quartz. 

From  the  extinction  angles  on  either  side  of  the  albite 
lamellae,  in  favorable  sections,  the  plagioclase  appears  to  lie 
between  a  basic  oligoclase  and  an  acid  labradorite. 

From  its  relations  to  the  quartz  and  feldspars,  the  horn- 
blende seems  to  have  been  the  first  mineral  in  the  order  of 
the  crystallization  of  the  essential  constituents  of  the  diorite. 
The  relations  of  quartz  and  feldspar  show  that  a  part  of  the 
feldspar  was  formed  before,  and  part  at  the  same  time  with 


GEOL.— VOL.  I.]         SMITH— SANTA  CATAL1NA  ISLAND.  17 

the  quartz.  The  simultaneous  development  with  the  quartz 
is  evidenced  by  a  frequent  irregular  intergrowth  along  the 
boundary  line  of  the  two  minerals,  and  further  by  the  occa- 
sional development  of  micropegmatitic  structure. 

Quartz  occurs  in  somewhat  smaller  individuals  than  do  the 
feldspars.  It  is  most  frequently  found  in  aggregations  or  in 
lines,  as  if,  being  the  last  mineral  to  form,  it  had  filled  the 
spaces  between  those  previously  existing.  It  is  present  in 
varying  amount  in  all  the  slides,  being  fairly  abundant  in 
some,  amounting  to  perhaps  one-fourth  of  the  total  minerals 
of  the  slide.  The  sections  vary  in  size  from  about  .1  mm. 
to  about  1.2  mm.  It  occurs  in  allotriomorphic  forms,  usu- 
ally with  very  irregular  outlines,  the  sections  being  fre- 
quently somewhat  intergrown  on  the  margins.  The  sections 
are  usually  clear.  Most  of  them  contain  liquid  inclusions, 
occurring  usually  without  any  definite  arrangement,  though 
occasionally  they  are  seen  in  lines  extending  through  several 
sections.  Besides  these  there  may  be  seen  with  the  higher 
powers  occasional  minute,  greenish  needles,  and  sections 
having  the  form  of  cross-sections  of  hornblende.  They 
are  without  noticeable  polarization.  The  quartz  also  con- 
tains occasional  magnetite. 

The  hornblende  occurs  in  sections  with  very  irregular 
boundaries,  due  to  resorption.  No  approach  to  crystal 
forms  was  seen.  The  feldspar  is  always  moulded  on  the 
hornblende,  except  in  one  case  observed.  In  this  section  a 
small  crystal  of  feldspar  was  seen  apparently  projecting  into 
one  side  of  the  hornblende,  the  feldspar  showing  good 
crystal  boundaries  where  surrounded  by  the  hornblende. 
Aside  from  this  instance  the  hornblendes  contain  no  inclu- 
sions of  feldspar,  while  the  feldspars  contain  occasional 
inclusions  of  hornblendic  material.  The  smaller  feldspars 
are  doubtless,  in  part,  at  least,  contemporaneous  with  the 
hornblendes,  though  the  feldspars  in  general  are  later.  In 
size  the  former  compare  favorably  with  the  feldspars. 
Twinning  parallel  to  the  orthopinacoid  is  common  in  the 
larger  and  fresher  sections.  The  pleochroism  is  pronounced, 

(  2  )  January  9,  1897. 


l8  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  3D  SER. 

c  being  green,  6  yellowish  brown,  and  a  pale  yellow-green 
to  almost  colorless.  The  absorption  formula  is  c  >  &  >  a. 
Inclusions  are  not  common  and  are  principally  magnetite. 
There  is  an  occasional  intergrowth  with  biotite.  The  alter- 
ation of  the  hornblende  is  well  advanced  in  many  of  the 
slides.  It  appears  to  be  undergoing  a  uralitic  change  by 
which  it  is  transformed  into  a  dirty  greenish,  fibrous  aggre- 
gate, much  like  "  reedy  hornblende,"  with  a  rather  weak 
pleochroism.  This  secondary  hornblende  has  usually  a 
parallel  arrangement  of  its  fibres,  and  the  terminals  of  the 
sections  are  generally  more  or  less  ragged.  It  also  occurs 
in  finely  fibrous,  irregular  areas,  with  the  fibres  irregularly 
oriented.  A  further  alteration  of  the  hornblende  is  mainly 
into  chlorite  and  calcite. 

Biotite  is  not  common,  though  occasionally  found.  The 
sections  are  strongly  pleochroic,  always  show  irregular 
boundaries,  and  the  mineral  occurs  either  alone  or  inter- 
grown  with  hornblende.  The  biotite  is  in  part  altered  to 
chlorite,  and  some  of  its  sections  are  wholly  surrounded  by 
a  chloritic  margin.  No  inclusions  occur,  except  occasional 
grains  of  magnetite. 

Augite  is  present  in  some  of  the  slides,  being  variable  in 
amount,  but  at  times  forming  an  important  constituent.  It 
generally  presents  very  irregular  boundaries,  but  several 
sections  were  seen  showing  roughly  the  crystal  form  charac- 
teristic of  cross-sections  of  augite.  It  has  a  granular,  much 
broken  appearance,  and  a  high  refractive  index.  Its  most 
characteristic  feature  is  a  clouding  of  the  area  by  an  opaque, 
dirty-brown  decomposition  product.  Few  of  the  sections 
were  free  from  this  product,  and  it  marked  the  mineral 
wherever  found.  The  augite  is  practically  colorless,  and 
is  without  any  sensible  pleochroism.  No  cleavage  was 
observed  anywhere,  and  only  one  case  of  twinning.  Where- 
ever  the  augite  comes  in  contact  with  the  hornblende  the 
boundary  line  is  sharp  and  clear.  When  it  occurs  in  isolated 
sections  these  are  usually  free  from  the  uralitic  product 
described  in  connection  with  the  hornblende.  This, 
together  with  the  freedom  of  all  the  hornblende  areas  from 


GEOL.— VOL.  I.]        SMITH—  SANTA  CATALINA  ISLAND.  19 

the  cloudy  decomposition  product  of  the  augite,  naturally 
leads  to  the  conclusion  that  none  of  the  fibrousjiqrnblende 
comes  from  the  augite. 

Magnetite  is  found  in  all  the  slides,  though  not  in  any 
considerable  amount.  It  occurs  as  inclusions  in  the  other 
minerals,  and  is  generally  in  the  form  of  grains  frequently 
showing  partial  crystal  boundaries.  In  size  they  range  up 
to  .3  mm.  Besides  the  grains,  there  are  several  very  irregu- 
lar areas  of  considerable  size — up  to  2  mm.  or  more  in 
length — in  or  near  the  areas  of  the  ferromagnesian  minerals. 

2.       PORPHYRITE. 

Following  the  usage  of  Iddings1  the  term  "porphyrite  " 
is  here  used  to  include  those  rocks  which  are  characterized 
by  a  medium-grained  porphyritic  structure,  and  which  con- 
tain among  their  essential  constituents  a  lime-soda-feldspar. 
They  constitute  the  connecting  link,  as  it  were,  between  the 
deep-seated  diorites  on  the  one  hand,  and  the  surficial 
andesitic  rocks  on  the  other,  and  pass  by  insensible  grada- 
tions into  either.  The  physical  conditions  attending  and 
controlling  its  crystallization  are  the  prime  factor  in  deter- 
mining the  position  of  the  rock  in  the  scheme  of  classifica- 
tion. 

Occurrence. — The  porphyrite  occurs  in  a  single  large  area 
in  the  southeastern  part  of  the  island,  and  was  not  found 
elsewhere  by  the  writer,  except  as  smaller  masses  in  the 
form  of  dikes.  The  main  area  has  an  average  wridth  of 
about  three  miles,  with  an  extreme  length  of  about  nine. 
It  is  cut  by  dikes  of  porphyrite  and  diorite,  from  two  to 
thirty  feet  or  more  in  width,  which  are  shown  on  the  cliffs 
at  a  number  of  places  along  the  shore. 

Character. — The  rocks  are  very  much  weathered,  and 
even  those  specimens  which  appeared  to  be  fairly  fresh 
were  seen,  when  examined  microscopically,  to  be  consider- 
ably altered.  In  weathering  the  rocks  first  break  into  coarse, 

1  Twelfth  An.  Kept.  U.  S.  Geol.  Surv.,  Part  I,  pp.  582-584. 


2O  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

irregular,  block-like  forms,  looking,  in  some  cases  along  the 
shore  near  Avalon,  like  square  pillars  projecting  from  the 
side  of  the  cliff.  These  break  up  into  smaller  block-like 
masses,  and  the  process  is  continued  until  the  gravelly  con- 
dition is  reached.  On  the  hill  slopes  the  projecting  masses 
frequently  present  similar  forms  with  smooth  surfaces,  but 
it  is  not  uncommon  to  see  also  small,  boss-like  projections, 
with  rough,  uneven  surfaces.  The  cause  of  this  difference 
in  form  is  doubtless  a  variation  in  the  grain  of  the  rock. 
The  soil  formed  from  these  rocks  is  generally  of  a  dull,  yel- 
lowish color.  The  porphyrite  contains  the  same  minerals 
that  occur  in  the  diorite,  except  biotite,  which  was  not  seen 
in  any  of  the  slides.  It  presents  the  same  general  charac- 
ters wherever  found. 

Macroscopic  Characters. — The  color  of  both  the  unaltered 
and  weathered  porphyrite  is  much  the  same  as  that  of  the 
diorite,  the  fresh  hand-specimens  varying  from  light  to  dark 
gray,  most  of  them  with  a  tinge  of  green.  Little  can  be  made 
out  in  the  fresher  specimens  with  the  unaided  eye,  except  an 
occasional  feldspar,  shown  by  the  reflection  from  a  cleavage 
surface.  Hornblende  crystals  of  some  length — up  to  5  mm. 
or  more — are  developed  in  one  or  two  specimens.  As  the 
rock  weathers  the  whitening  of  the  feldspars  usually  brings 
out  plainly  the  porphyritic  structure.  With  the  lens  the  por- 
phyritic  feldspars  may  occasionally  be  distinguished  from 
the  medium-grained  ground-mass,  though  these  are  usually 
masked  more  or  less  by  the  fracture  of  the  rock,  which 
leaves  minute  flakes  or  splinters  clinging  to  the  surface  of 
the  specimen.  The  rock  frequently  presents  a  slightly 
mottled  surface,  in  the  dark  and  light  colors.  Gleaming 
bits  of  pyrite  may  occasionally  be  seen  in  some  of  the 
specimens. 

Microscopic  Characters. — Microscopically  the  rock  is  holo- 
crystalline  and  porphyritic,  with  phenocrysts  of  a  lime-soda- 
feldspar  and  of  hornblende  —  occasionally  also  of  augite  — 
and  a  medium-grained  granular  ground-mass,  composed  es- 
sentially of  feldspar  and  quartz.  The  phenocrysts  vary  con- 


GEOL.— VOL.  I.]         SMITH— SANTA  CATALINA  ISLAND.  21 

siderably  in  number.  In  some  slides  there  are  comparatively 
few,  while  in  others  they  constitute  the  larger  portion  of  the 
slide.  The  ground-mass  is  never  glassy,  and  does  not  show 
any  flow  structure,  except  to  a  slight  extent  in  specimens 
from  one  or  two  dikes.  Quartz  is  seen  sometimes  among 
the  phenocrysts,  though  this  is  a  very  rare  occurrence. 
Magnetite  occurs  as  an  accessory,  but  usually  in  very  small 
amounts.  No  apatite  was  seen,  and  in  the  rock  of  which 
the  analysis  is  given  below  no  phosphorus  was  found. 
Most  sections  show  little  evidence  of  disturbance. 

The  porphyritic  feldspars  occur  in  idiomorphic  forms, 
which  are  somewhat  tabular  parallel  to  the  brachypinacoid. 
The  majority  of  the  crystals  show  good  boundaries,  except 
where  their  growth  was  interfered  with  by  the  growth  of 
other  phenocrysts.  Many  of  the  sections,  however,  show 
boundaries  which  are  more  or  less  irregular  or  rounded, 
and  due,  in  part,  at  least,  to  resorption.  The  sections  vary 
in  size  from  about  .2  mm.  to  nearly  3  mm.  Zoning  is 
common.  The  twinning  is  in  accordance  with  the  albite 
and  Carlsbad  laws.  Pericline  twinning  rarely  occurs.  As 
in  the  diorite,  the  feldspar  lies  between  a  basic  oligoclase 
and  an  acid  labradorite.  Many  of  the  sections  are  con- 
siderably cracked.  Some  of  the  feldspars  are  fairly  fresh, 
but  most  of  them  are  more  or  less  clouded  by  decomposi- 
tion products.  This  cloudiness  is  due  largely  to  a  kaolini- 
zation  of  the  mineral.  Considerable  areas  are  sometimes 
altered  to  calcite,  with  more  or  less  epidote.  The  decom- 
position is  such  at  times  as  to  destroy,  partly  or  wholly, 
the  traces  of  twinning.  Occasional  inclusions  of  hornblende 
or  chlorite  are  seen  in  the  feldspars.  One  section  was 
seen  with  a  zone  of  chloritic  material  not  far  from  the  bound- 
ary, arranged  in  threads  and  fibres  parallel  to  the  longer 
direction  of  the  feldspar.  Small  magnetite  grains  are  rarely 
included  in  the  feldspar. 

The  hornblende  is  prismatic  in  habit,  and  usually  either 
shows  resorbed  boundaries,  or  the  original  outlines  are  more 
or  less  obliterated  by  alteration  products.  No  terminal  planes 
were  seen  in  any  section.  In  two  or  three  of  the  slides 


22  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

the  characteristic  lozenge-shaped  cross-sections  were  seen, 
with  a  distinct  prismatic  cleavage.  Twinning  is  common, 
parallel  to  the  orthopinacoid.  Comparatively  few  unaltered 
sections  were  found,  most  of  the  hornblende  which  was 
originally  present  in  the  slides  having  been  altered  to  chlo- 
rite and  calcite.  In  some  of  the  slides  the  hornblende  is 
wholly  replaced  by  secondary  minerals.  The  freshest  sec- 
tions are  frequently  surrounded  by  a  chloride  border,  or 
decomposition  has  begun  along  cracks  and  cleavage  planes. 
The  pleochroism  of  both  chlorite  and  hornblende  is  the 
same  as  in  the  diorites.  The  hornblende  in  some  cases  has 
the  fibrous  character  described  under  the  diorites.  In  one 
slide  in  which  this  fibrous  hornblende  occurs,  without  good 
crystal  boundaries,  there  is  another  secondary  hornblende, 
with  different  optical  properties,  and  having  the  form  char- 
acteristic of  augite. 

From  what  has  been  said  it  will  be  seen  that  hornblende 
is  a  primary  constituent  of  the  rock.  It  is  also  the  dom- 
inant ferromagnesian  mineral.  Roughly  estimated,  it  con- 
stitutes, with  its  decomposition  products,  about  one-fourth 
or  one-third  of  the  total  amount  of  minerals  in  the  slides. 

Augite  is  found  in  varying  amounts  in  nearly  all  the  slides, 
usually  in  the  form  of  irregular  grains,  or  as  irregular 
brownish  or  more  or  less  opaque  patches.  It  has  the  same 
general  structure  and  habit  as  in  the  diorite  and  its  decom- 
position products  are  similar.  Usually  it  has  a  very  insig- 
nificant position  compared  with  the  other  minerals  of  the 
rock.  In  two  or  three  of  the  slides,  however,  it  compares 
in  amount  with  the  hornblende,  and  in  one  slide  in  particu- 
lar— from  a  specimen  from  the  canon  back  of  Avalon — the 
sections,  though  rather  small,  are  quite  numerous,  constituting 
perhaps  one-fifth  or  one-fourth  of  the  total  minerals  of  the 
slide.  Here  the  mineral  is  fairly  fresh  and  free  from  decom- 
position products. 

It  is  almost  colorless  or  pale  green,  and  without  pleochro- 
ism. When  any  crystal  boundaries  are  shown  they  are 
only  partial.  The  form  when  developed  shows  the  usual 
octagonal  cross-section.  The  crystals  occur  as  separate 


GEOL.— VOL.  I.]        SMITH— SANTA  C ATA  UNA  ISLAND.  23 

individuals.  The  feldspars  are  molded  on  the  augites, 
and  sometimes  completely  enclose  the  smaller  sections. 
The  augite  shows  no  definite  cleavage,  but  is  usually  more 
or  less  traversed  by  cracks.  In  this  slide  no  hornblende 
was  seen,  though  there  are  considerable  areas  of  chlorite. 
No  doubt  hornblende  was  at  one  time  present,  being  now 
represented  by  the  chlorite,  for  the  augite  in  the  slide  has 
no  border  of  chlorite,  nor  is  the  latter  seen  along  the  cracks 
of  that  mineral.  When  near  or  touching  chloritic  areas  the 
augites  have  as  sharp  boundaries  and  appear  quite  as  fresh 
as  those  sections  which  have  no  chlorite  near  them.  In 
weathering  the  augite  alters  to  a  granular,  dirty  brownish 
product,  more  or  less  opaque.  In  one  slide,  from  the 
northern  side  of  Avalon  canon,  what  was  originally  augite 
with  a  short  prismatic  habit,  is  entirely  altered  to  hornblende 
(see  page  22) .  The  sections  are  idiomorphic,  with  the  forms 
characteristic  of  augite,  but  with  the  cleavage  of  horn- 
blende .  In  vertical  sections  the  cleavage  is  very  pronounced , 
showing  in  part  as  open  cracks.  This  hornblende  is  pleo- 
chroic,  a  being  a  very  pale  yellow,  fc  pale  yellowish  green, 
c  greenish  brown.  The  mineral  is  more  or  less  dull  in 
appearance,  and  the  polarization  colors  are  not  clear.  It  is 
quite  unlike  the  fibrous  hornblende  in  character.  One  of 
the  sections  shows  indistinct  twinning  lamellae  parallel  to 
the  orthopinacoid.  The  augites  contain  as  inclusions  occa- 
sional magnetite  grains. 

The  magnetite  varies  greatly  in  amount,  being  almost 
entirely  absent  from  some  of  the  sections.  In  the  rock 
containing  abundant  augite  there  is  considerable  magnetite 
in  small,  irregular  patches  or  needle-like  forms,  at  times  in 
or  cutting  across  the  feldspars,  or  projecting  into  the  augites. 
One  slide  from  Pebbly  Beach  shows  a  few  small  grains  with 
the  crystal  form  of  magnetite,  but  altered  to  limonite. 

The  granular  ground-mass  of  the  porphyrite  is  composed 
of  usually  allotriomorphic  feldspar  and  quartz,  the  larger 
proportion  being  of  the  former.  At  times,  however,  the 
feldspars  tend  to  lath-shaped  or  rectangular  forms.  The 
borders  of  the  grains  in  the  ground-mass  frequently  inter- 


24  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

lock.  Occasionally  small,  ragged  flakes  of  hornblende  or 
small  patches  of  chlorite  occur.  The  minerals  of  the 
ground-mass,  in  those  slides  showing  the  most  pronounced 
porphyritic  structure,  have  a  diameter  of  from  .03  mm.  to 
.1  mm.  As  the  size  of  the  grains  increases  the  rock 
assumes  the  structure  of  diorite-porphyrite,  with  occasional, 
though  rare,  porphyritic  quartzes.  All  gradations  were 
found  between  porphyrite  and  diorite. 

The  quartz  varies  considerably  in  amount  in  the  porphy- 
ritic rocks.  Usually  it  is  rather  subordinate,  but  occasionally 
it  is  quite  abundant,  forming  perhaps  one-fourth  of  the 
minerals  of  the  slide.  These  rocks,  however,  are  not 
common,  and  are  doubtless  only  local  developments.  One 
slide  of  the  diorite-porphyrite  is  remarkable  for  the  manner 
in  which  the  quartz  is  developed  with  respect  to  the  other 
minerals.  This  rock  (from  the  slopes  to  the  west  of  the 
entrance  to  Silver  Canon)  contains  abundant  phenocrysts 
of  feldspar,  with  porphyritically  developed  quartz.  Under 
crossed  nicols  the  quartz  appears  as  scattered  and  more  or 
less  rounded  grains.  Occasionally  several  of  these  are 
found  near  together,  showing  similar  polarization  colors  and 
a  common  extinction.  On  revolving  the  stage  it  is  seen  that 
the  rounded  borders  do  not  mark  the  limit  of  the  sections 
of  quartz;  for  the  extinction  of  the  mineral  shows  that  it 
has  an  outer  zone  which  has  a  pronounced  micropoikilitic 
structure,  being  closely  packed  with  finely  polarizing  feld- 
spars. The  quartz  occasionally  shows  a  crystal  form.  The 
ground-mass  of  the  rock  is  coarsely  crystalline  and  consists 
largely  of  micropoikilitic  quartz  similar  to  the  larger  sections, 
but  without  clear  centers.  A  very  few  of  the  larger  sections 
also  are  wholly  filled  with  feldspar  aggregates.  The  clear 
centers  together  with  the  micropoikilitic  margins  indicate 
arrested  development  of  the  quartz,  which  began  to  form 
before  the  growth  of  the  minute  feldspars,  the  latter  form- 
ing before  the  final  crystallization  of  the  quartz.  The  rock 
is  much  altered,  and  except  in  one  or  two  instances  the 
traces  of  twinning  in  the  feldspar  phenocrysts  are  wholly 


GEOL.-VOL.  I.]        SMITH—  SANTA  CATALINA  ISLAND.  25 

obliterated    by    decomposition    products.     Hornblende    is 
wholly  replaced  by  chloritic  material. 

Analysis.  —  The  porphyrite  was  nowhere  found  in  an  en- 
tirely fresh  condition.  The  following  analysis  was  made 
from  the  freshest  specimen  obtained,  as  shown  by  its  thin 

section: 

I.  II. 

SiO2  ............................  63.82  65.71 

Ti  O2.  .  .  .    .......................  trace 

A12  O3  ...........................  16.53  i7-°8 

Fe2  O3  ...........................   1.28  2.84 

Fe  O  ............................  2.93  1.79 

MnO  ...........................  trace 

Ca  0  ............................  5-57  5-24 

Mg  O  ............................   1.99  2-57 

Na2O  ...........................  4.12  3.87 

K2O  .............................  77  1-02 

H2  0  ............................  1.82 


C  O  ............................   1.  10 


99.93  100.12 

Sp.  gr  ............   ..............  2.689 

I.     Porphyrite  from  Pebbly  Beach,  Santa  Catalina. 
II.     Quartz-diorite,  Dognaska.     (Banatite.) 

The  analysis  of  the  porphyrite  differs  but  little  from  that 
of  the  banatite  given  above.  In  some  of  its  aspects  the 
diorite  is  not  much  unlike  the  microscopic  character  of 
some  of  the  banatites. 

Inliers  of  Basement  Rocks.  —  At  a  number  of  points  in  the 
porphyrite  area,  following  the  general  direction  of  the  main 
ridge  to  the  west  of  Avalon,  and  along  a  line  extending 
from  the  coast  northwest  of  Avalon  to  near  the  coast  east  of 
Silver  Canon,  there  occur  at  intervals  patches  of  the  base- 
ment rocks.  These  are  found  not  only  along  the  main  crest, 
but  on  several  of  the  branch  ridges  to  the  west  and  north- 
west of  Avalon,  and  on  one  of  the  branches  running  into 
Silver  Canon.  They  also  occur  at  irregular  intervals  at  the 
base  of  the  cliffs,  from  Avalon  Harbor  for  a  distance  of 
about  a  mile  to  the  northwest.  The  outcrops  along  the 
ridges  vary  from  a  few  yards  to  nearly  200  feet  in  length. 


26  CALIFORNIA  ACADEMY  OF  SCIENCES.    [PROC.  3D  SER. 

Only  the  largest  have  been  indicated  on  the  map  (even 
those  being  necessarily  made  on  a  larger  scale  than  that  of 
the  map),  where  they  are  shown  as  quartzite,  though  also 
including  rocks  from  all  the  basement  series.  Those  in  the 
cliff  sections  occur  as  definite  inclusions,  and  vary  in  length 
from  a  fraction  of  an  inch  to  about  50  feet.  The  porphyrite 
in  other  places  also  contains  inclusions,  though  nowhere  are 
they  so  abundant  as  at  the  points  mentioned. 

It  is  possible  that  some  of  the  outcrops  along  the  ridges 
are  remnants  of  a  former  covering  to  the  porphyrite,  but 
undoubtedly  some  (if  not  all)  of  them  exist  as  inclusions 
within  the  igneous  rock,  as  do  the  occurrences  along  the 
shore ;  for  some  of  them  occur  in  saddles  along  the  main 
ridge,  while  those  on  the  minor  ridges  are  in  great  part 
hundreds  of  feet  below  the  average  altitude  of  the  main 
ridge.  To  contain  such  large  inclusions  an  intrusive  of 
considerable  size  must  be  predicated.  This,  together  with 
the  size  and  form  of  the  area  of  the  porphyrite,  points  to 
its  origin  as  a  laccolite.1  The  microscopical  character  of 
the  rock,  together  with  its  mode  of  occurrence,  clearly 
indicates  its  intrusive  nature. 

The  mass  of  the  porphyrite  appears  to  be  roughly  dome- 
shaped,  with  a  somewhat  elliptical  base,  and  though  no 
remnant  of  a  cover  was  found  it  cannot  be  doubted  that  one 
formerly  existed,  now  removed  by  extensive  and  active  ero- 
sion. The  base  of  the  mass  was  not  seen  at  any  point. 

Porphyrite  Dikes. — Occasional  dikes  penetrate  both  the 
porphyrite  area  and  that  of  the  quartzite,  the  latter  at  the  west 
end  particularly.  At  the  mouth  of  Silver  Canon  the  diorite, 
also,  is  cut  by  porphyrite  dikes,  one  of  which  contains  numer- 
ous inclusions  of  the  diorite.  Wherever  the  directions  of  the 
dikes  could  be  determined  they  were  found  to  be  nearly  ver- 
tical, or  within  20°  of  the  perpendicular,  and  approximately 

1The  term  "laccolite"  is  here  used  in  the  sense  of  a  somewhat  dome-shaped  mass 
which  has  been  intruded  into  a  yielding  body  of  rock,  not  necessarily  along  the  bedding 
planes.  On  this  view  the  undisturbed  condition  of  the  beds  previous  to  the  intrusion  is 
of  minor  importance,  the  main  factors  being  the  possession  of  basement  and  cover,  and 
the  dome-shaped  form  of  the  mass. 


GEOL.— VOL.  I.]         SMITH— SANTA  CATALINA  ISLAND.  2>J 

parallel,  the  range  being  from  N.  25°  E.  to  N.  65°  E.  Prob- 
ably all  of  these  dikes  are  of  nearly  the  same  age  as  the 
porphyrite,  though  many,  or  possibly  most  of  them,  are  a 
little  later,  judging  from  the  fact  that  the  area  of  the  por- 
phyrite itself  is  penetrated  by  them.  Most  of  the  specimens 
obtained  are  very  much  altered,  and  contain  a  comparatively 
large  amount  of  calcite,  but  there  is  enough  of  the  original 
structure  left  to  show  definitely  that  the  rocks  are  porphyrite. 
In  general  nothing  further  than  this  could  be  determined, 
though  most  of  the  specimens  appear  to  be  not  much  unlike 
the  rocks  of  the  main  mass. 

A  somewhat  different  structure  is  shown  in  two  or  three 
of  the  slides,  only  one  of  which  will  be  described.  This  is 
from  a  dike  on  the  northern  coast,  at  a  point  about  a  third 
of  the  distance  from  the  isthmus  to  the  extreme  northwest- 
ern end  of  the  island.  The  dike  is  nearly  vertical,  and  has 
a  width  of  eight  feet.  Specimens  were  taken  from  near  the 
margin  and  central  portion  of  this  dike,  the  two  being  en- 
tirely different  in  appearance.  That  from  near  the  center 
is  a  pale,  even  gray,  while  the  other  is  a  darker,  mottled 
gray,  with  an  intergrowth  of  light  and  dark  areas.  The 
darker  parts  appear  to  be  compressed  in  a  given  plane,  and, 
as  seen  with  a  lens,  have  usually  a  minute  central  cavity. 
The  rock  from  the  middle  of  the  dike  also  contains  here  and 
there  very  minute  thread-like  cavities. 

Although  the  hand-specimens  differ  so  much  in  general 
appearance  the  contrast  is  not  so  great  under  the  microscope. 
Phenocrysts  are  not  very  numerous  and  are  wholly  of  labra- 
dorite,  which  is  quite  fresh.  They  are  considerably  resorbed, 
and  seldom  show  crystal  boundaries.  The  ground-mass  is 
almost  wholly  filled  with  lath-shaped  feldspar  microlites  of 
various  sizes,  which  show  a  pronounced  flow  structure  in 
the  slide  of  the  marginal  rock,  this  being  less  noticeable  in 
the  specimen  from  the  center.  A  majority  of  these  micro- 
lites have  indented  terminals.  In  the  marginal  rock  slide 
there  are  two  distinct  types  of  areas  in  the  ground-mass,  in 
both  of  which  microlites  occur,  one  somewhat  yellowish,  and 
the  other  dark  in  color,  from  minute  particles  contained  in 


28  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

it.  In  the  first  type  the  finer  portion  of  the  ground-mass  has 
a  microcrystalline  structure.  In  the  other  case  the  matrix 
is  largely  isotropic,  and  is  principally  of  secondary  silica  in 
the  form  of  opal.  These  latter  areas  have  usually  a  small 
irregular  central  cavity  somewhat  rounded  or  oblong  in 
shape.  In  some  cases  these  centers  have  been  filled  with 
secondary  quartz.  These  darker  areas  of  secondary  silica 
are  usually  separated  from  the  lighter  ones  by  an  irregular 
and  generally  narrow  band  of  a  yellowish  green,  showing 
high  polarization  colors.  Under  crossed  nicols  and  with 
higher  powers,  this  is  seen  to  be  composed  of  finely  polar- 
izing aggregates,  doubtless  of  some  secondary  ferromagne- 
sian  mineral.  Similar  minute,  radial  aggregates  occur 
scattered  through  the  lighter  areas  of  the  slide. 

The  slide  from  the  central  portion  of  the  dike  does  not 
show  the  division  into  light  and  dark  areas,  though  it  con- 
tains a  small  amount  of  opaline  silica  and  quartz.  There  is 
a  larger  amount  of  the  greenish  yellow  ferromagnesian 
mineral,  which  is  more  evenly  distributed  through  the  rock, 
a  part  in  the  form  of  radial  aggregates  and  a  part  as  minute 
flakes.  The  matrix  is  cryptocrystalline,  showing  a  feeble 
polarization.  The  ground-mass  is  filled  with  opaque,  dust- 
like,  microscopic  particles. 

3.     RHYOLITE. 

Occurrence. — The  rhyolite  occurs  in  a  single  area,  to 
the  west  of  the  main  area  of  the  andesite,  in  the  Little 
Harbor  region.  It  caps  the  summit  of  the  ridge  at  this 
point,  and  extends  as  a  light  covering  to  the  basement  rocks 
for  several  hundred  feet  down  the  southern  and  western 
slopes,  the  underlying  formations  appearing  here  and  there. 
At  a  point  about  midway  down  the  western  slope  the  rock 
has  a  roughly  bedded  appearance,  dipping  toward  the  west 
at  a  rather  high  angle.  The  relation  of  these  rocks  to  the 
other  igneous  rocks  of  the  island  was  not  learned. 

Macroscopic  Characters. — The  rhyolite  varies  from  com- 
pact to  very  vesicular,  and  is  of  a  light  color,  nearly  white 


GEOL.— VOL.  I.]         SMITH— SANTA   CATALINA  ISLAND.  29 

or  with  a  tinge  of  pink.  It  appears  to  be  considerably 
altered.  It  contains  scattered  phenocrysts  of  quartz^  (up  to 
3  mm.  in  diameter)  with  smaller  and  more  numerous  crystals 
of  biotite.  The  vesicles  indicate  flow  by  their  pronounced 
compression  in  one  plane.  Some  of  the  cavities  have  a 
smooth,  lustrous  surface,  and  appear  to  be  regular  in  shape, 
as  if  due  to  the  leaching  out  of  phenocrysts  once  contained 
in  them.  The  form  of  two  or  three  of  these  cavities  strongly 
suggested  a  simply  twinned  feldspar. 

Microscopic  Characters. — In  thin  section  the  open-textured 
facies  of  the  rock  is  seen  to  be  composed  largely  of  a  dirty 
brown,  very  vesicular  ground-mass,  in  which  occur  scattered 
phenocrysts  of  quartz  and  biotite,  besides  more  or  less  mag- 
netite in  small  grains  or  crystals.  No  phenocrysts  of  feld- 
spar were  seen,  but  one  cavity  was  found  which  clearly 
had  the  form  of  a  Carlsbad  twin  of  feldspar.  This  cavity 
had  a  very  narrow  border  of  some  secondary  product  strongly 
stained  with  limonite. 

The  quartz  is  in  general  quite  clear,  and  occurs  in  idio- 
morphic  forms  which  are  usually  more  or  less  corroded. 
Frequent  cracks  traverse  the  sections.  Besides  brownish 
patches  of  included  glass,  the  quartz  contains  occasional 
small  spherulites,  and  sections  of  biotite  partly  or  wholly 
included.  No  liquid  inclusions  were  seen. 

Biotite  occurs  in  scattered,  idiomorphic  sections,  generally 
with  clear  boundaries.  A  few  of  the  rectangular  sections 
are  somewhat  frayed  at  the  ends.  The  crystals  range  in 
length  from  .15  mm.  to  .7  mm.  The  mineral  exhibits  the 
usual  strong  pleochroism. 

The  ground-mass  consists  in  large  part  of  feebly  polarizing 
feldspar  microlites  in  a  dark  isotropic  matrix.  A  few  of  the 
vesicular  cavities  of  the  slides  are  nearly  round,  the  rest 
being  elliptical  in  form,  and  occasionally  drawn  out  at  the 
ends.  Some  contain  a  small  amount  of  a  clear,  secondary 
mineral,  and  others,  nearly  spherical,  are  completely  filled 
with  almost  opaque  secondary  products,  dirty  brown  to  black 
in  color. 


30  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

4.     ANDESITE. 

Occurrence. — There  is  one  main  area  of  the  andesitic 
rocks  with  several  smaller  occurrences.  These  rocks  are 
all,  both  macroscopically  and  microscopically,  identical,  and 
undoubtedly  indicate  an  originally  continuous  area,  cover- 
ing the  larger  part  of  the  eastern  division  of  the  island. 

Ascending  the  ridge  next  the  ocean,  to  the  south  of  Mid- 
dle Ranch  Canon,  this  rock  is  first  met  with  at  an  altitude 
of  about  300  feet,  where  it  forms  a  small  patch  extending 
from  this  point  to  an  elevation  of  about  500  feet.  The  next 
area  occurs  at  an  altitude  of  about  1,100  feet,  where  the 
rock  not  only  forms  the  summit  of  this  part  of  the  ridge  but 
caps,  as  well,  a  minor  ridge  which  extends  into  the  adjacent 
canon.  The  third  occurrence  is  near  the  head  of  this  canon. 
Besides  these  more  definite  areas,  the  soil  at  a  number  of 
places  in  this  region  has  a  purplish  tinge,  and  the  general 
appearance  points  to  a  more  extended  areal  distribution  of 
the  andesite.  Erosion  has,  however,  entirely  removed  the 
rock  in  some  places,  while  in  others  it  has  left  only  the  thin- 
nest coating  on  the  rocks  beneath,  or  the  former  covering 
remains  simply  as  a  coloring  to  the  soil,  in  places  occupied 
by  other  rocks  which  normally  weather  to  a  yellow. 

There  is  a  small  area  of  andesite  at  the  extreme  south- 
eastern end  of  the  island.  Here  much  of  the  andesite  con- 
tains inclusions  in  varying  amounts,  the  rock  in  some  places 
being  well  filled  with  this  fragmental  material,  which  is  de- 
rived in  large  part  from  earlier  andesitic  flows. 

The  main  area  of  the  andesite  has  a  general  easterly  and 
westerly  trend,  and  extends  from  the  shore  on  the  north 
side  to  the  lower  slopes  south  of  Little  Harbor.  It  reaches 
an  extreme  altitude  of  2,109  ^eet-  ^  consists  of  a  series  of 
volcanic  flows  which  present  a  distinct  banding  on  the  face 
of  the  cliffs  northwest  and  southeast  of  Swain's  Landing. 
These  bands  have  a  width  of  four  or  five  feet  and  upwards. 
To  the  east  of  Swain's  Landing  they  have  a  dip  of  io°-i2° 
toward  the  Landing,  while  on  the  other  side  they  dip  in  the 
opposite  direction  and  at  an  angle  of  about  3°.  Midway 


GEOL.— VOL.  I.]         SMITH— SANTA  CAT  A  LIN  A  ISLAND.  31 

between  this  point  and  Whitley's  Cove  the  bands  are  more 
or  less  irregularly  flexed,  though  preserving  a  general  par- 
allelism to  the  shore-line. 

Another  small  area  of  these  rocks  is  found  along  the  coast 
to  the  east  of  Empire  Landing. 

The  area  of  andesite  near  the  isthmus  is  also  formed  by  a 
series  of  flows  which  show  a  distinct  banding  along  the  cliff 
on  the  ocean  side.  This  banding  preserves  a  course  rough- 
ly parallel  to  the  water-line  till  near  Isthmus  Cove,  where  it 
changes  its  direction,  dipping  at  an  angle  of  about  25°  to- 
ward the  point  at  the  entrance  to  the  cove,  as  shown  in  the 
section  on  the  map.  This  formation  reaches  its  greatest 
altitude  near  the  southeastern  end,  where  it  is  about  900 
feet  above  sea-level.  Along  the  northern  shore-line  of  this 
area  adjoining  Isthmus  Cove  numerous  faults  are  seen  (not 
shown  on  the  map),  ranging  in  throw  from  a  few  inches 
to  a  hundred  feet  or  more. 

The  most  marked  feature  of  this  area  is  a  distinct  white 
band  following  the  upper  line  of  the  cliff  for  some  distance, 
and  overlying  the  volcanic  rocks.  This  is  the  bed  of  tuff 
and  diatomaceous  earth  already  mentioned  as  occurring  with 
the  volcanics  here.  That  the  andesite  lies  above  as  well  as 
below  it  is  plainly  seen  at  a  number  of  points.  An  espe- 
cially good  section  showing  the  upper  contact  of  the  tuff  is 
obtained  in  the  little  bay  to  the  east  of  Isthmus  Cove.  Here 
the  tuff  is  overlain  by  porous  andesitic  rocks. 

The  rocks  along  this  part  of  the  shore  have  been  hollowed 
out  in  places  by  the  force  of  the  waves,  forming  caves,  pil- 
lars, and  blow-holes. 

The  coarse  banding  of  the  andesites  along  the  cliff  sec- 
tions shows  a  variety  of  colors,  the  rocks  weathering  in  dark 
grayish  or  purplish  with  occasional  reddish  tints.  The  soil 
formed  by  this  series  of  rocks  is  always  purplish  in  color, 
and  is  easily  distinguished,  even  at  a  distance,  from  the  soils 
which  the  other  rocks  of  the  island  form,  the  latter  being 
either  reddish  or  yellowish. 

Macroscopic  Characters. — The  freshest  specimens  of  the 
andesite  are  black  or  nearly  so.  Though  the  rocks  are  in  gen- 


32  CALIFORNIA  ACADEMY  OF  SCIENCES.        [Pnoc.  30  SER. 

eral  dull,  some  of  them  have  an  almost  greasy  luster.  A  few 
of  the  specimens,  purplish  in  color,  appear  to  the  eye  to  be 
fresh  and  compact,  but  with  a  lens  it  may  generally  be  seen 
that  they  are  more  or  less  altered.  Though  usually  compact, 
the  rocks  are  vesicular  in  places,  the  irregular  vesicles  more 
or  less  compressed  in  the  plane  of  flowage.  This  was  noticed 
particularly  about  Isthmus  Cove;  also  in  places  back  of 
Whitley's  Cove.  Rarely  the  rock  is  amygdaloidal,  as  in 
the  neighborhood  of  Whitley's  Cove.  The  rock,  though 
usually  fracturing  irregularly,  at  times  breaks  into  plate-like 
pieces  a  centimeter  or  more  in  thickness.  These  pieces 
may  sometimes  be  broken  into  thinner  plates,  owing  to  a 
laminated  condition  of  the  rock.  These  latter  sheets  vary 
in  thickness  from  two  millimeters  to  several  centimeters, 
and  their  surfaces  are  generally  yellowish  from  decomposi- 
tion products.  In  other  cases  the  rocks  break  into  irregular 
masses,  while  showing  a  phenomenon  similar  to  the  fore- 
going in  a  series  of  fine  parallel  lines  on  those  fractured 
surfaces  at  right  angles  to  the  bedding.  One  and  only  one 
glassy  specimen  of  the  rock  was  obtained,  from  near  the 
small  bay  to  the  east  of  Isthmus  Cove. 

Microscopic  Characters. — Microscopically  the  rocks,  with 
the  exception  of  two  of  the  specimens  examined,  are  pyrox- 
ene-andesites.  They  are  usually  porphyritic  with  a  hyalopil- 
itic  ground-mass.  The  phenocrysts  consist  essentially  of 
labradorite,  augite,  and  hypersthene.  Secondary  silica  is 
usually  present  in  greater  or  less  amounts.  This  is  largely 
opal  with  occasional  chalcedony.  The  first  mineral  to  sep- 
arate from  the  magma  was  magnetite,  followed  by  the  pyrox- 
enes, and  finally  by  the  feldspars. 

The  magnetite  occurs  either  as  small  octahedrons  or  in 
irregular  patches,  and  appears  to  be  in  two  generations. 
The  largest  grains  are  about  .2  mm.  in  diameter.  These 
are  not  very  numerous.  The  smaller  grains  are  more  abun- 
dant, and  are  distributed  more  or  less  evenly  through  the 
ground-mass.  These  have  an  average  diameter  of  about  .04 
mm.  This  scattered  magnetite  forms  one  of  the  most  marked 


GEOL.— VOL.  I.]        SMITH— SANTA  CAT  A  LIN  A  ISLAND.  33 

features  of  several  slides  which  have  very  few  phenocrysts. 
It  amounts  in  one  to  perhaps  one-third  as  much  as  the  feld- 
spars of  that  slide,  and  equals  or  very  slightly  exceeds  the 
amount  of  pyroxene.  The  magnetite  occurs  as  inclusions 
in  all  the  other  phenocrysts. 

The  augite  is  usually  idiomorphic,  though  where  not  well 
developed  as  phenocrysts  it  occurs  as  minute  flakes.  It  is 
very  pale  green,  almost  colorless,  and  without  noticeable 
pleochroism.  Its  habit  is  prismatic  and  the  resulting  forms 
are  generally  octagonal.  The  crystal  outlines  are  usually 
very  sharp  and  clear,  though  rounded  and  resorbed  sections 
are  not  uncommon.  The  cleavage,  in  general,  is  not  visible 
except  with  higher  powers.  Cracks  are  common,  traversing 
the  crystal  in  every  direction.  The  augites  generally  are 
remarkably  clear  and  free  from  alteration  products.  A  few 
of  the  larger  sections,  however,  are  much  dulled  and 
cracked.  Twinning  parallel  to  the  orthopinacoid  is  com- 
mon. Liquid  inclusions  are  numerous,  occasionally  reaching 
a  diameter  of  .02  mm.  Some  inclusions  of  magnetite  also 
occur. 

The  hypersthene  differs  but  little  from  the  augite  in  gen- 
eral appearance,  in  form,  habit,  inclusions,  or  its  relation  to 
the  essential  minerals,  and  cannot  always  be  readily  distin- 
guished from  it.  The  former,  however,  is  very  slightly 
pleochroic,  and  all  sections  give  parallel  extinction.  It  has 
a  somewhat  weaker  double  refraction  than  the  augite,  and 
the  interference  colors  of  the  sections  are  therefore  in  general 
lower,  showing  yellow  of  the  first  order  in  the  majority  of 
cases.  Further,  favorable  sections  give  characteristic  inter- 
ference figures.  Though  its  habit  is  like  that  of  the  augite, 
the  prismatic  faces  occasionally  are  little  developed,  or 
rarely  are  entirely  wanting.  The  two  minerals  are  occa- 
sionally intergrown. 

Whenever  the  two  pyroxenes  are  developed  as  definite 
phenocrysts  the  hypersthene  is  always  in  excess  of  the 
augite.  The  number  of  the  pyroxene  phenocrysts  in  the 
rocks  is  always  much  smaller  than  that  of  the  feldspars,  the 
ratio  being  about  one  to  four  or  five. 

(  3  )  January  9,  1897. 


34  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  3D  SER. 

The  feldspar  phenocrysts  occur  in  idiomorphic  sections 
and  are  usually  lath-shaped,  presenting  a  tabular  develop- 
ment parallel  to  the  brachypinacoid.  There  is  a  slight  tend- 
ency to  the  formation  of  ruin-like  terminals.  The  crystal 
boundaries  are  in  many  cases  clear  and  sharp,  though  most 
of  them  show  a  varying  amount  of  resorption  both  on  the 
sides  and  terminals  of  the  sections,  oftener  the  latter.  Some 
of  the  feldspars  clearly  show  a  second  period  of  growth 
after  having  been  in  part  resorbed  into  the  magma.  Zoning 
is  rather  common.  Twinning  is  in  accordance  with  both 
albite  and  Carlsbad  laws.  Occasional  cracks  penetrate  the 
sections  and  some  of  these  are  brought  out  more  clearly  by 
a  staining  of  limonite.  In  sections  cut  approximately  per- 
pendicular to  the  albite  lamellation  the  extinction  angles 
show  the  species  to  be  labradorite.  As  a  rule  the  sections 
of  the  feldspar  are  very  fresh  and  clear,  being  free  from 
decomposition  products.  Inclusions  are  not  uncommon, 
consisting  chiefly  of  brownish  glass  with  a  few  small  and 
irregular  pyroxenes  and  occasional  small  grains  of  magnet- 
ite. The  glass  is  usually  in  irregular  patches  and  is  either 
centrally  or  zonally  arranged.  In  some  cases  it  occurs  in 
roughly  rectangular  forms  zonally  arranged,  and  with  their 
longer  axis  parallel  to  the  longer  axis  of  the  crystal.  The 
length  of  the  sections  of  labradorite  varies  from  perhaps 
.06  mm.  to  about  1.5  mm.  The  sections  are  usually  quite 
numerous,  though  their  number  is  very  variable,  and  some 
of  the  specimens  show  few  phenocrysts  of  any  sort. 

The  phenocrysts  of  the  various  minerals  generally  occur 
as  scattered  crystals,  but  they  occasionally  form  small  ag- 
gregates. In  this  case,  the  boundaries  of  the  feldspars  yield 
to  those  of  the  ferromagnesian  minerals  with  which  they 
are  in  contact.  This,  with  the  occasional  complete  inclu- 
sion of  these  minerals  already  mentioned,  shows  the  later 
development  of  the  feldspars. 

The  ground-mass  consists  largely  of  feldspar  microlites 
with  a  varying  amount  of  interstitial  glass.  Pyroxene  oc- 
curs occasionally  in  flakes  in  the  ground-mass,  when  but  lit- 
tle developed  porphyritically.  The  feldspars  have  a  low 


GEOL.— VOL.  L]         SMITH— SANTA    CAT  A  LIN  A  ISLAND.  35 

extinction  angle,  most  of  them  extinguishing  parallel  to  their 
length,  thus  placing  them  in  the  oligoclase-andesine  series. 
Many  of  them  appear  to  be  simply  twinned.  In  length  they 
range  up  to  about  .04  mm.  In  nearly  all  cases  they  show  a 
general  parallelism  due  to  flow.  The,  amount  of  glass  in 
the  ground-mass  is  very  variable,  some  of  the  slides  being 
almost  holo crystalline.  In  other  cases  the  glass  and  second- 
ary silica  compose  nearly  one-half  the  ground-mass.  This 
glass  is  readily  distinguished  from  the  opal  by  its  color. 
The  latter  is  yellowish  brown  and  is  quite  clear,  while  the 
glass  is  dark  and  usually  filled  with  small,  irresolvable  dots 
which  may  be  magnetite. 

Glassy  Fades. — The  glassy  facies  of  the  andesite,  found 
as  a  very  small  occurrence  near  the  isthmus,  differs  both 
macroscopically  and  microscopically  from  that  just  described. 
The  rock  is  dark,  almost  black,  and  glassy,  with  yellowish 
patches  scattered  over  the  surface,  the  largest  seen  being 
13  mm.  in  length.  Occasional  kaolinized  feldspars  occur, 
either  within  these  yellowish  patches  or  alone,  up  to  a  length 
of  about  2  mm.  With  a  lens  very  small  hexagonal  crystals 
of  biotite  are  seen  here  and  there.  The  dark  ground-mass 
constitutes  the  bulk  of  the  rock. 

Under  the  microscope  scattered  and  aggregated  crystals 
of  magnetite,  hypersthene,  augite,  biotite,  and  feldspar  are 
seen  here  and  there  in  a  glassy  matrix.  Magnetite,  in  grains 
and  small  octahedra,  occurs  as  inclusions  in  the  other  min- 
erals, and  scattered  through  the  ground-mass.  There  is 
only  a  moderate  amount  of  biotite  in  the  slides.  It  occurs 
as  isolated  idiomorphic  sections,  and  was  seen  nowhere  in 
contact  with  the  other  minerals.  No  basal  sections  were 
seen.  The  mineral  everywhere  exhibits  the  usual  strong 
pleochroism. 

Augite  occurs  either  alone  or  with  hypersthene,  and  while 
generally  more  or  less  rounded,  it  occasionally  shows  good 
crystal  boundaries.  It  is  pale  green  in  color  and  non-pleo- 
chroic.  One  section  of  the  augite  showing  rough  crystal 
boundaries  is  wholly  surrounded  by  a  somewhat  rectangular 


36  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

growth  of  hypersthene,  the  augite  occupying  a  nearly  cen- 
tral position.  The  vertical  axes  of  the  two  minerals  are 
similarly  oriented.  The  augite  is  simply  twinned,  the  two 
halves  of  the  section  showing  a  very  slight  difference  of 
extinction.  The  hypersthene  is  much  fractured  and  pre- 
sents the  appearance  of  a  number  of  rods  placed  in  parallel 
position,  side  by  side  and  end  to  end.  The  interstices 
between  the  rod-like  parts  are  filled  with  a  brownish  yellow 
isotropic  substance  closely  resembling  the  opaline  silica  in 
the  slides  already  described.  This  mineral  also  forms  an 
intricate  network  in  most  of  the  feldspars,  and  constitutes  a 
considerable  proportion  of  many  of  the  yellowish  patches 
which  are  so  numerous  in  the  rock.  These  yellowish  areas 
are  largely  aggregates  of  hypersthene  and  feldspar. 

The  hypersthene  is  prismatic  in  habit  and  occurs,  in  gen- 
eral, either  alone  or  in  aggregations  together  with  occa- 
sional feldspars.  The  feldspar  and  hypersthene  are  the 
most  abundant  minerals  of  the  slide,  the  former  being  some- 
what in  excess  of  the  latter.  The  hypersthene  has  a  pro- 
nounced pleochroism,  c  being  light  green  in  color,  6  reddish 
brown,  and  a  very  pale  reddish.  The  absorption  formula 
is  c>B>a. 

In  one  slide  two  small  sections  of  free  quartz  were 
seen  occurring  quite  close  together  and  showing  rounded 
and  somewhat  corroded  boundaries.  The  sections  are  both 
crossed  by  numerous  cracks. 

The  feldspars  are  all  much  resorbed,  and  seldom  show 
crystal  boundaries.  The  brownish  network  which  most  of 
them  contain  is  frequently  central,  leaving  a  narrow  border 
free  from  inclusions. 

The  ground-mass,  with  a  high  power,  is  seen  to  consist  of 
glass  filled  with  crystallites  and  microlites,  with  here  and 
there  perlitic  cracks.  With  a  low  power  the  crystallites 
and  microlites  appear  only  as  dusty  particles.  The  perlitic 
cracks  are  distributed  very  irregularly,  some  portions  of  the 
slides  being  entirely  free  from  them.  They  are  usually 
quite  numerous  near  the  larger  aggregates  of  the  pheno- 
crysts,  and  frequently  those  bordering  these  areas  are  stained 


GEOL.— VOL.  I.]        SMITH— SANTA  CAT  A  LIN  A  ISLAND.  37 

yellow.  Such  stained  cracks  show,  under  crossed  nicols,  a 
faint  polarization,  as  if  from  some  radially  arranged  sec- 
ondary product.  The  microlites  are  capillary  in  form, 
averaging  .01  mm.  in  length,  and  exhibit  a  slight  polariza- 
tion. They  also  show  a  pronounced  flow  structure.  In 
smaller  amount  are  the  margarites  and  trichites,  the  latter 
in  the  form  of  tufts  and  wisps,  and  sometimes  curled  at  the 
ends.  The  margarites,  also,  are  occasionally  gathered  into 
loose  tufts,  radiating  in  all  directions. 

Basaltic  Fades. — It  was  stated  (page  32)  that  the  speci- 
mens examined  were  all  pyroxene  andesites,  with  but  two 
exceptions.  One  of  these  was  found  on  the  minor  ridge 
bounding  the  valley  of  Whitley's  Cove  on  the  north,  not  far 
from  the  contact  between  the  basement  rocks  and  the  vol- 
canics.  The  other  is  from  the  slopes  to  the  east  of  Isthmus 
Cove.  The  rocks  differ  from  those  already  described,  in 
containing  iddingsite.1  The  specimen  from  near  the  isthmus 
is  dark  gray  in  color  and  very  much  altered,  but  the  iddings- 
ite occurs  here  in  good  crystal  forms,  while  in  the  other 
specimen,  which  is  fairly  fresh,  the  crystal  boundaries  show 
more  or  less  resorption.  As  the  structure  of  the  latter  rock 
differs  considerably  from  that  usually  found  in  the  andesites, 
it  will  be  described  in  some  detail. 

The  rock  is  purplish  and  compact,  but  passes  into  a  black 
vesicular  facies,  apparently  differing  microscopically  from 
the  compact  form  only  in  the  size  of  the  component  minerals 
and  in  the  amount  of  magnetite  contained.  The  minerals 
are  much  smaller  in  the  vesicular  portion  of  the  specimen, 
and  it  is  almost  black  with  magnetite,  in  the  form  of  grains, 
long,  roughly  bordered  rods,  and  irregular  areas.  The 
glassy  ground-mass  of  this  portion  of  the  rock  is  filled  with 
minute  black  dots,  doubtless  magnetite.  The  compact  por- 
tion of  the  specimen  has  very  little  glass,  though  it  contains 
a  large  amount  of  secondary  silica,  chiefly  in  the  form  of 
opal.  The  rock  is  nearly  holocrystalline,  and  contains 

1  See  "Geology  of  Carmelo  Bay,"  by  Andrew  C.  I^awson.  Bull.  Dept.  Geol.,  Univ. 
Cal.,  Vol.  i,  pp.  31-36. 


38  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

very  fresh,  lath-shaped  feldspars  in  two  generations,  besides 
amber-colored  iddingsite  and  pale  green  augite.  The 
specific  gravity  is  2.770. 

The  opal  has  the  usual  appearance  of  that  mineral  macro- 
scopically.  On  one  face  of  the  specimen  there  is  a  con- 
siderable crust  of  hyalite  showing  a  distinctly  botryoidal 
surface  when  viewed  with  a  lens.  It  is  colorless  and  nearly 
transparent,  with  a  vitreous  luster  and  a  hardness  of  about 
5.5.  It  is  infusible,  dissolving  in  soda,  with  effervescence, 
to  a  clear  glass.  It  is  in  large  part  soluble  in  caustic  pot- 
ash, and  in  the  closed  tube  gives  water.  The  areas  of 
opal  in  the  slides  are  all  isotropic.  The  sections  readily 
take  a  stain  after  heating  with  concentrated  hydrochloric 
acid, which,  however,  scarcely  attacks  the  powdered  mineral. 
The  color  of  the  opal  in  thin  section  is  light  brown. 
Cavities  occur  in  it,  occasionally  lined  with  chalcedony. 

Both  generations  of  feldspars  appear  to  be  labradorite, 
and  the  larger  ones  form  the  most  prominent  feature  of  the 
slide,  being  rather  numerous  and  of  considerable  size, 
ranging  in  length  from  about  2  mm.  to  3.5  mm.  They  are 
allotriomorphic,  and  contain,  as  inclusions,  considerable 
iddingsite,  besides  a  little  glass,  an  occasional  smaller 
feldspar  in  the  largest  sections,  and  rarely  augite.  The 
small  included  feldspars  always  show  more  or  less  resorp- 
tion. 

The  augite  occurs  in  small  grains  with  very  irregular 
boundaries.  These  contain  many  cracks  which  give  them 
a  granular  appearance.  The  mineral  shows  no  alteration 
and  contains  as  inclusions  occasional  grains  of  magnetite, 
besides  partially  included  small  feldspars.  Several  sections 
were  seen  with  a  few  small  feldspars  wholly  enclosed. 

The  most  characteristic  mineral  of  the  rock  is  the  iddings- 
ite. In  amount  it  slightly  exceeds  the  augite,  and  equals 
about  one-third  of  the  feldspar.  It  varies  in  size  from  .06 
mm.  to  nearly  .4  mm.  It  was  the  second  mineral  to  separate 
from  the  magma,  preceded  by  the  magnetite.  It  occurs  as 
usually  elongated  grains  with  very  irregular  boundaries, 
frequently  marked  by  bays  due  to  magmatic  corrosion. 


GEOL.— VOL.  I.]         SMITH— SANTA  CATALINA  ISLAND.  39 

Where  the  original  form  is  indicated  it  is  very  similar  to  that 
of  olivine.  The  form  is  best  shown  in  the  rock  from  near 
the  isthmus.  That  rock,  however,  is  very  soft  and  much 
altered,  and  no  satisfactory  microscopic  sections  of  the 
mineral  were  obtained,  except  for  the  determination  of  the 
outline.  Several  small  but  good  crystals  were  made  out  in 
the  rock,  with  a  lens.  As  seen  thus,  the  general  form  tallies 
with  that  described  by  Prof.  Lawson.1  The  mineral  here 
is  deep  brownish  red  in  color,  with  a  pronounced  cleavage, 
the  cleavage  surfaces  presenting  a  somewhat  metallic  luster. 
The  central  portion  of  the  crystals  is  usually  dark  green. 
The  forms  which  the  slides  present  are  of  two  types,  both 
hexagonal,  one  with  a  pronounced  cleavage,  the  other  with- 
out a  cleavage  but  with  a  distinct  fibration  at  right  angles  to 
the  direction  of  elongation.  The  cleavage  subtends  an 
angle,  two  measurements  of  which  gave  131°  and  133°, 
respectively.  Referring  the  mineral  to  the  same  system  of 
axes  to  which  Prof.  Lawson  has  referred  it,  a  is  perpendic- 
ular to  the  cleavage,  b  in  the  cleavage  and  parallel  to  the 
fibration,  c  in  the  plane  of  cleavage  and  perpendicular  to  b. 

The  cleavage  is  well  shown  by  a  series  of  parallel  and 
narrow,  open  seams,  to  which  the  extinction  is  in  all  cases 
parallel.  In  sections  in  which  the  cleavage  is  wanting  the 
extinction  is  always  parallel  to  the  longer  direction  of  the 
section  and  to  the  fibration.  The  latter  sections  show  a 
fair  biaxial  interference  figure.  The  mineral  is  therefore 
orthorhombic.  The  emergence  of  the  acute  bisectrix  is 
perpendicular  to  the  plane  of  cleavage  and  b— £.  The 
optical  character  of  the  mineral,  as  determined  by  means  of 
the  quartz  wedge,  is  negative,  a  is  therefore  the  acute  bisec- 
trix, and  =«;  c=£. 

The  color  of  the  mineral  in  thin  section  is  very  variable, 
especially  in  different  parts  of  one  and  the  same  section, 
ranging  from  a  golden  brown  to  a  clear  though  not  bright 
yellow,  with  occasional  dull  greenish  areas  in  or  near  the 
center.  The  deeper  colors  are  usually  marginal  or  along 


40  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

the  frequent  cracks,  and  appear  to  be  due  to  limonite  formed 
by  oxidation  of  the  contained  iron.  In  all  the  attempts  made, 
however,  this  color  could  not  be  leached  out  by  acids.  The 
deep  color  of  the  sections  and  strong  absorption  of  light 
prevented  an  entirely  satisfactory  determination  of  some  of 
the  optical  properties. 

The  pleochroism  is  marked  in  sections  transverse  to  the 
cleavage,  but  is  not  so  strong  in  sections  showing  no  cleav- 
age. The  absorption  formula  is  c  >  6  >  a.  The  mineral 
possesses  a  rather  low  mean  index  of  refraction.  In  the 
thinner  sections  the  iddingsite  may  be  seen  to  possess  a 
strong  double  refraction,  though  the  polarization  colors  are 
usually  masked  by  the  deep  color  of  the  mineral.  In  sec- 
tions parallel  to  the  plane  of  cleavage,  though  the  transverse 
fibration  parallel  to  b  is  distinct,  the  color  of  the  mineral 
conceals  the  fibration  at  right  angles  to  this.1 

No  satisfactory  material  could  be  obtained  for  investi- 
gating the  mineral  chemically,  nor  was  any  attempt  made 
to  analyze  the  rock  as  a  whole,  on  account  of  the  secondary 
silica  contained  in  it.  One  of  the  slides  was  uncovered  and 
an  attempt  was  made  to  stain  the  mineral.  This  was  suc- 
cessful only  after  several  trials  had  been  made,  both  with 
concentrated  and  dilute  acids.  Dilute  boiling  sulphuric 
acid  finally  caused  the  mineral  to  take  the  stain,  the  results 
thus  agreeing  with  those  obtained  by  Dr.  Ransome.2 

No  definite  information  was  obtained  from  the  Santa 
Catalina  specimens  as  to  the  origin  of  this  mineral.  Its  oc- 
currence in  a  rock  of  this  type,  and  possessing  the  form 
characteristic  of  olivine,  would  certainly,  in  the  absence  of 
any  evidence  to  the  contrary,  point  to  the  strong  probability 
of  its  being  a  pseudomorph  after  that  mineral.  If  the  min- 
eral described  by  Iddings3  is  the  same  as  that  under  discus- 
sion— as  it  appears  to  be — it  leaves  little  doubt  on  the  ques- 
tion. 

iSee  "The  Eruptive  Rocks  of  Point  Bonita,"  by  F.  Leslie  Ransome.  Bull.  Dept. 
Geol.,  Univ.  Cal.,  Vol.  i.  No.  3,  p.  91. 

*Loc.  cit.,  p.  92. 

8 "Geology  of  the  Eureka  District,  Nevada."  Monongraph  XX,  U.  S.  G.  S.,  Appen- 
dix B,  pp.  388-390. 


GEOL.— VOL.  I.]         SMITH— SANTA  CATALINA  ISLAND.  41 

Analysis. — The  following  analysis  was  made  from  a 
particularly  fresh  specimen  of  the  andesite,  obtained  from 
the  ridge  forming  the  eastern  boundary  of  the  valley  back 
of  Swain's  Landing: 

Si  O2 61.05 

TiO2 09 

A12O8 * 18.30 

Fe203 3-49 

Fe  O i. ii 

Mn  O trace 

CaO 7.75 

Mg  O 2.59 

Na2  0 4.06 

K20 1.36 

H20 71 

P2  O5 trace 


100.51 
Sp.gr 2.668 

5.     RELATIVE  AGE. 

The  porphyrite  was  nowhere  found  in  a  fresh  condition, 
while  the  andesite  at  many  points  is  very  fresh.  The  pro- 
nounced difference  in  the  amount  of  alteration  which  the 
two  rocks  have  undergone  would  suggest  that  the  andesite 
is  the  younger.  When  the  feldspars  of  the  two  are  com- 
pared this  difference  amounts  to  more  than  a  mere  sugges- 
tion. These  feldspars  are  closely  related  chemically,  and, 
other  things  being  equal,  those  of  the  older  rock  should 
show  a  greater  amount  of  weathering.  In  the  fresher  an- 
desite the  feldspars  are  remarkably  free  from  decompo- 
sition products,  while  those  of  the  porphyrite  always  show 
a  greater  or  less  degree  of  alteration.  In  none  of  the  spec- 
imens do  the  feldspars  compare  with  those  of  the  andesite 
in  freshness. 

More  positive  evidence  as  to  the  relative  age  of  the  two 
rocks  was  obtained  on  the  western  slopes  of  the  andesites, 
in  the  Little  Harbor  region.  Here,  at  an  elevation  of  about 
1,200  feet,  numerous  inclusions  of  the  porphyrite  are  found 
in  the  andesite.  They  are  of  very  irregular  shape,  averag- 
ing two  or  three  inches  in  diameter.  Differential  weathering 


42  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

frequently  causes  them  to  stand  out  on  the  surface  of  the 
rock.  These  inclusions  show  beyond  a  doubt  that  the  por- 
phyrite  is  the  older  of  the  two  rocks. 

B.     TUFF  AND  DIATOMACEOUS  EARTH, 
i .     OCCURRENCE. 

This  material  has  already  been  mentioned  as  occurring 
intercalated  with  the  andesites  of  the  isthmus  region  near 
their  upper  limits,  and  forming  a  single  composite  bed  of 
considerable  thickness.  So  far  as  known  there  is  but  this 
one  occurrence.  Though  there  are  excellent  exposures  to 
the  east  of  Isthmus  Cove  no  complete  section  of  the  bed 
was  seen,  but  from  the  several  parts  it  is  estimated  to  have 
a  thickness  of  from  one  hundred  to  two  hundred  feet. 

Besides  this  bed  on  the  island,  there  is  doubtless  a  consid- 
erable deposit  of  similar  material  just  outside  of  Isthmus 
Cove,  as  indicated  by  the  sounding  contours.  The  large 
scale  Coast  Survey  map  of  the  isthmus  emphasizes  this, 
and  shows,  by  mapping  in  the  contours,  a  more  or  less  con- 
tinuous submarine  ridge,  extending  out  some  distance. 
This  ridge  is  marked  in  its  course  by  a  shoaling  of  the 
water  at  one  point,  and  by  two  small  islands.  These  islands 
are  within  the  200  ft.  and  300  ft.  contours,  and  are  29  feet 
and  66  feet  in  height,  respectively.  The  nearer  of  the  two 
is  composed  of  tufaceous  material,  and  the  outer  one  is 
probably  of  the  same,  though  it  was  not  seen  near  at  hand. 

The  bed  of  the  island  is  not  homogeneous  but  is  composed 
of  numerous  minor  beds  of  varying  thickness  and  color, 
now  of  the  white  shale  and  now  of  reddish,  yellowish,  or 
dark  grayish  tufaceous  material.  The  thickness  of  a  given 
bed  is  seldom  the  same  for  any  distance,  and  it  frequently 
happens  that  a  bed  will  wedge  out  and  disappear  within  a 
few  rods.  One  of  the  beds  noted  changes  in  thickness 
from  about  six  feet  to  one  foot  in  a  little  more  than  fifty  feet. 
The  various  beds  of  the  formation  therefore  occur  in  no 
fixed  order,  though  the  bulk  of  the  deposits  is  always  of  the 
diatomaceous  earth. 


GEOL.— VOL.  I.]        SMITH— SANTA  CATALINA  ISLAND.  43 

2.      TUFF. 

The  tufaceous  beds  are  not  wholly  of  volcanic  material, 
but  contain  more  or  less  of  the  shale  fragments,  besides 
having  occasionally  a  matrix  of  the  shale  (diatomaceous 
earth).  They  also  contain  fragments  of  the  metamorphic 
material  similar  to  that  composing  the  underlying  basement 
rocks.  The  rock  as  a  whole  is  rather  soft,  and  usually 
somewhat  compact.  One  of  the  highest  beds  is  made  up 
of  moderately  fine  angular  material  and  is  somewhat  porous, 
as  it  has  very  little  cementing  material.  Usually  fragmental 
shaly  material  appears  to  form  most  of  the  finer  portion  of 
the  rock.  Some  of  the  beds  are  more  or  less  even-grained, 
composed  wholly  of  moderately  fine  material,  and  contain- 
ing nothing  larger  than  half  an  inch  in  diameter.  Others 
contain,  besides  this,  large  angular  blocks,  the  largest  attain- 
ing a  length  of  about  two  and  a  half  feet,  though  most  of 
them  are  less  than  a  foot  and  a  half  in  length.  By  the 
weathering  of  the  face  of  the  cliffs  many  of  these  blocks 
project  from  the  surface.  In  this  way  both  the  larger  and 
smaller  fragments  gradually  work  out  and  fall  to  the  base  of 
the  cliff.  These  blocks  are  composed  almost  wholly  of 
very  vesicular  and  usually  much  reddened,  andesitic  lava. 
A  few  large  fragments  of  shale  were  seen  in  these  coarser 
beds  and  a  number  of  angular  blocks  of  the  metamorphic 
material,  some  of  them  a  foot  and  a  half  in  length.  These 
beds,  are  in  places  fossiliferous,  and  it  is  said  that  large 
pectens  have  been  found  in  them.  No  fossils  were  found 
by  the  writer,  however. 

3.     SHALE. 

The  shale  wherever  found  is  white  or  light  gray  in  color, 
but  it  varies  considerably  in  texture  and  composition.  As  a 
rule  the  rock  is  very  soft  and  earthy,  and  can  be  easily 
scratched  with  the  nail.  In  this  condition  it  has  a  low 
specific  gravity.  The  more  earthy  and  less  compact  the 
rock,  the  more  easily  it  splits  into  thin  sheets.  The  lightest 


44  CALIFORNIA  ACADEMY  OF  SCIENCES.         [PROC.  30  SER. 

separates  into  paper-like  fragments,  almost  thin  enough  for 
microscopic  sections.  The  rock  is  found  in  two  other  con- 
ditions, one  opaline  and  the  other  calcareous,  between 
which  and  this  type  there  are  all  gradations.  Both  these 
less  common  types  are  hard  and  compact,  and  are  quite 
brittle,  breaking  with  a  conchoidal  fracture.  The  opaline 
variety  has  a  hardness  of  about  5,  and  in  places  has  a  luster 
like  that  of  opal.  This  is  No.  89,  described  by  Dr.  Hinde 
on  page  48.  The  calcareous  rock  is  somewhat  darker  in 
color  than  the  others.  It  effervesces  quite  freely  with  strong 
acids,  while  with  dilute  acids  it  behaves  like  dolomite.  The 
gradation  of  this  rock  into  the  light,  earthy  shale  is  seen  by 
testing  the  different  specimens  with  acid.  The  different 
grades  of  the  rock  show  different  degrees  of  effervescence, 
while  the  most  earthy  specimens  give  apparently  none.  The 
opaline  variety  does  not  effervesce  with  acid.  The  effer- 
vescence is  due  in  many  of  the  specimens  to  minute  cal- 
careous remains,  but  in  the  darker  rock  it  results  from  the 
calcium-magnesium  carbonate  which  makes  up  the  mass  of 
the  rock.  All  the  specimens  give  water  in  the  closed  tube. 
Heated  they  turn  black,  then  white,  giving  off  bituminous 
odors.  The  specific  gravity  of  the  earthy  specimens  could 
not  be  determined  on  account  of  their  porous  character. 
That  of  the  limestone  is  2.69. 

No  fossils  of  any  considerable  size  were  found  in  these 
rocks.  Some  of  the  more  siliceous  specimens  contain 
minute,  empty  molds,  arranged  along  the  bedding  planes. 
In  one  of  the  more  compact  specimens  two  fragmentary 
shell  casts  were  seen,  besides  a  small  cast  of  what  is  prob- 
ably Tellina  congesta,  Conrad.  Fish-scales  are  quite  com- 
mon, with  their  delicate  markings  well  preserved. 

Microscopic  Characters. — Under  the  microscope  the  shale 
is  seen  to  consist  largely  of  isotropic  material,  in  which  are 
scattered  angular  crystal  fragments.  The  isotropic  portions 
of  the  slides  appear  to  be,  not  of  glass,  but  wholly  (or  nearly 
so)  of  organic  remains.  The  crystal  fragments  vary  in  amount 
in  different  specimens,  or  even  in  different  parts  of  a  single 
slide,  but  on  the  whole  they  form  but  a  small  percentage  of 


GEOL.— VOL.  I.]        SMITH— SANTA  CAT  A  LIN  A  ISLAND.  45 

the  entire  rock.  They  are  largely  microscopic  in  size,  with 
here  and  there  a  larger  fragment  reaching  an  extreme  length 
of  about  .1  mm.  This  fragmental  material  is  largely  of 
feldspar  some  of  which  shows  twinning,  besides  an  occa- 
sional dull  greenish  patch  of  chloritic  material.  A  few 
quartz  fragments  occur,  but  the  source  of  these  may  be  the 
quartzite  of  the  basement  series,  as  several  fragments  were 
seen  in  one  of  the  slides,  composed  of  very  small  inter- 
locking grains  of  quartz.  Most  of  these  fragmental  crystals 
show  a  feeble  polarization,  particularly  the  smaller  ones. 
In  one  of  the  slides  were  seen  several  larger  fragments  of 
andesite,  somewhat  altered,  but  still  fresh  enough  to  show 
the  twinning  of  the  porphyritic  feldspars. 

A  section  of  the  calcareous  rock  shows  that  it  is  appar- 
ently free  from  the  angular  fragments  of  the  shaly  speci- 
mens, while,  as  before,  the  mass  of  the  rock  appears  to 
consist  of  organic  remains.  With  crossed  nicols  the  larger 
molds  are  seen  to  be  filled  with  calcite.  The  ground-mass 
of  the  rock  is  not  isotropic,  but  gives  the  delicate  polariza- 
tion tints  of  calcite.  With  a  high  power  the  entire  rock  is 
seen  to  have  a  microcrystalline  structure,  being  made  up  of 
irregular  grains  of  calcite.  This  structure  bears  no  rela- 
tion to  the  distribution  of  the  organic  material  in  the  rock, 
except  in  the  case  of  the  Foraminifera. 

Character  of  the  Organic  Remains. — A  number  of  small 
fragments  of  the  shale  were  forwarded  by  Prof.  Lawson  to 
Dr.  George  J.  Hinde  for  examination.  He  has  kindly 
placed  his  conclusions  at  Prof.  Lawson's  disposal,  in  the 
following  note. 

"From  small  samples  of  these  rocks  sent  over  to  me  by 
Prof.  A.  C.  Lawson  I  have  prepared  thin  microscopic  sec- 
tions where  the  material  was  sufficiently  coherent  to  allow 
of  such  being  made,  and  in  the  case  of  the  very  soft  rocks 
the  fine  powder  has  been  mounted  just  as  it  occurs,  without 
washing  away  the  finer  debris.  I  have  only  aimed  in  the 
following  notes  at  giving  a  general  idea  of  the  nature  of  the 
organisms  of  which  the  rocks  are  composed,  for  the  task  of 
recognizing  even  the  genera  present  would  prove  too  long 


46  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

and  difficult  to  be  undertaken,  and  moreover,  the  material 
at  hand,  though  sufficient  to  show  the  general  character  of 
the  organisms,  would  not  be  enough  for  determination  of 
particular  forms. 

11  No.  90.  This  soft,  white,  earthy  rock  is  essentially 
diatomic  in  character.  Both  in  section  and  in  powder  it  is 
seen  to  consist  of  a  mass  of  heterogeneously  mingled  frag- 
ments of  diatom  frustules,  with  a  small  proportion  of  com- 
plete forms.  By  far  the  larger  mass  of  the  rock  is  formed 
by  the  broken  up  and  disintegrated  particles  of  the  diatoms, 
and  the  smallest  and  finest  portions  recognizable  under  the 
microscope  are  clearly  organic  debris.  Coscinodiscus  appears 
to  be  the  predominant  genus.  Detached  sponge  spicules  are 
fairly  numerous.  They  are  principally  pin-shaped  and 
styliform;  also  a  few  simple  fusiform  rods  occur  belonging 
to  the  Monactinellid  division  of  siliceous  sponges.  The 
Tetractinellid  sponges. are  represented  by  a  few  fragmentary 
trifid  spicules  and  globate  forms.  Only  one  or  two  some- 
what doubtful  fragments  of  Radiolaria  were  noticed,  and 
these  organisms  must  have  been  very  sparsely  present,  for 
their  structures  are  stouter  and  more  capable  of  preservation 
than  the  diatoms.  The  silica  of  these  organisms — diatoms 
and  sponges — appears  to  be  unaltered  in  the  fossilization — 
it  retains  the  same  glassy  aspect  as  in  recent  examples.  In 
addition  to  the  siliceous  organisms,  Foraminifera  are  like- 
wise present,  and  they  yet  retain  the  calcareous  structure  of 
their  walls,  though  hardly  so  well  preserved  as  in  the  case 
of  the  siliceous  fossils.  A  rather  large  form  of  Textularia 
is  the  most  common  of  the  Foraminifera.  It  is  to  these 
organisms  that  the  calcareous  portions  of  the  rock  are  due. 
The  rock  is  very  finely  laminated,  showing  a  series  of  well 
marked  undisturbed  layers  of  organic  remains  in  which  are 
scattered  some  minute  angular  chips  of  minerals  here  and 
there. 

"No.  118.  A  whitish,  comparatively  soft,  earthy  rock. 
Examined  both  in  section  and  in  powder.  Very  similar  to 
the  preceding  in  consisting  nearly  wholly  of  diatoms  and 
diatomic  debris.  Coscinodiscus  is  very  numerous;  some 


GEOL.— VOL.  I.]         SMITH— SANTA  CATALINA  ISLAND.  47 

forms  relatively  large.  Sponge  spicules  are  also  present, 
but  I  could  not  certainly  distinguish  any  Radiolaria.  No 
Foraminifera  to  be  seen  in  this  specimen,  and  there  was  no 
reaction  of  the  rock  in  acid.  It  is  not  unlikely  that  the 
calcareous  organisms  have  been  leached  away,  for  minute 
empty  pores  can  be  seen  in  transverse  sections  of  the  beds. 
It  has  a  lesser  proportion  of  angular  rock  chips  than  the 
preceding  (No.  90). 

"  No.  147.  Very  soft,  earthy  white  rock,  readily  break- 
ing up  into  fine  flaky  laminae.  No  reaction  in  acid.  Like 
the  preceding  this  is  also  nearly  entirely  a  diatomic  rock, 
but  the  diatoms  are  here  of  different  forms,  Melosira?  and 
Grammatophora  being  most  conspicuous.  There  are  fair 
numbers  of  sponge  spicules,  usually  broken;  they  are  chiefly 
pin-shaped  and  styliform.  Neither  Foraminifera  nor  Radio- 
laria were  recognizable  in  the  material  examined.  The 
angular  rock  chips  were  fewer  in  this  rock  than  in  the  pre- 
vious specimens  (90  and  118). 

"  No.  I52.1  A  pale  gray,  hard  rock — just  scratches  with 
knife — compact,  flinty  fracture,  readily  effervesces  in  acid. 
Examined  in  section  only.  It  consists,  like  the  soft  rock 
above  referred  to  (No.  90),  mainly  of  diatoms  and  diatomic 
debris;  the  ground-mass  of  the  rock  is,  as  far  as  can  be  seen 
under  the  microscope,  wholly  of  the  broken  up  diatom 
frustules.  Both  the  minute  fragments  and  the  entire  forms 
are  as  unchanged  as  in  the  soft  rocks.  Coscinodiscus  is 
abundant,  also  Navicula,  Grammatophora  and  other  forms. 
Some  of  the  spaces  between  the  diatom  frustules  have  been 
infilled  with  calcite.  There  are  a  few  Radiolaria  present, 
spheroidal  and  discoidal  forms,  but  their  numbers  are  insig- 
nificant in  comparison  with  the  diatoms.  Sponge  spicules 
are  apparently  absent.  Foraminifera  are  fairly  common  and 
well  preserved,  showing  their  wall  structures;  the  most 
abundant  is  a  large  species  of  Textularia,  probably  the  same 
form  as  that  in  No.  90.  The  interiors  of  the  Foraminifera 
have  been  infilled  with  calcite.  Angular  chips  hardly  to  be 
seen  in  the  sections  of  this  rock. 

1  No.  152  is  the  limestone,  the  analysis  of  which  is  given  later,  p.  50. 


48  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

"  No.  89.  Pale  gray  or  cream-tinted  hard  rock — just 
scratches  with  knife — no  action  in  acid.  The  section 
examined  showed  numerous  minute  pores,  but  whether  these 
indicated  spaces  where  organisms  had  been  is  doubtful.  No 
organisms  could  be  recognized  in  this  rock,  which,  never- 
theless, appears  to  be  of  opalized  silica.  A  few  angular 
chips  could  be  distinguished  in  polarized  light. 

"  With  the  exception  of  this  last  specimen,  the  siliceous 
and  silico-calcareous  rocks  of  the  island  of  Santa  Catalina 
are  remarkable  for  the  very  slight  amount  of  alteration 
which  the  structures  of  the  siliceous  and  calcareous  organ- 
isms have  undergone  in  the  fossilization.  Both  the  most 
delicate  diatoms  and  the  Foraminifera  occur  in  these  beds 
together,  in  nearly  as  well  preserved  condition  as  in  deposits 
now  forming.  The  beds  may  well  be  compared  with  recent 
diatomic  oozes,  and,  as  in  these  latter,  there  is  a  small  per- 
centage of  sponge  spicules,  Radiolaria  and  Foraminifera 
mingled  with  the  prevailing  diatoms.  The  paucity  of  Radi- 
olaria in  the  beds  is  a  peculiar  feature.  Sections  of  these 
rocks  show  very  distinctly  that  the  entire  material,  down  to 
the  smallest  particles,  is  of  organic  remains  mostly  now 
broken  up,  for  the  proportion  of  perfect  forms  is  small 
compared  with  the  large  quantity  of  fragmental  debris. 
The  amount  of  the  foreign  angular  mineral  particles  is  insig- 
nificant." 

Chemical  Characters. — This  note  would  seem  to  leave  no 
room  for  question  as  to  the  organic  origin  of  the  shale,  but 
that  the  point  might  be  considered  from  all  sides,  a  chemical 
determination  was  made  of  the  amount  of  soluble  silica  in  the 
most  earthy  and  least  calcareous  of  the  specimens.  About  a 
a  gram  of  the  roughly  powdered  material  was  used,  in  a  ten 
per  cent,  solution  of  potassium  hydrate.  For  the  piirpose  of 
comparison  specimens  of  pumice  and  nearly  pure  volcanic 
ash  were  taken  and  subjected  to  the  same  treatment  as  the 
shale.  All  the  material  was  well  dried  at  100°  (C)  before 
weighing  and  adding  to  the  solution.  The  solutions  were 
brought  to  boiling  twice,  being  allowed  to  stand  some  hours 
in  the  interval,  and  for  about  a  day  after  the  second  heating. 


GEOL.-VOL.  I.]        SMITH— SANTA  CATALINA  ISLAND.  49 

The  residues  were  then  filtered  off,  and  the  silica  was  precip- 
itated in  the  filtrate  by  acidifying  with  hydrochloric  acid,  and 
evaporating  to  dryness.  The  weight  of  the  silica  obtained 
by  this  process  was  compared  with  the  weight  of  the  resi- 
dues, and  except  for  the  shale  they  all  tallied  very  closely. 
All  lost  some  silica,  and  on  the  addition  of  ammonia  after 
the  precipitation  of  the  silica  a  slight  amount  of  alumina 
was  precipitated  in  all  the  solutions,  showing  that  the  alumina 
in  the  rock  was  acted  on  to  some  extent.  The  same  test 
applied  to  the  potassium  hydrate  (which  occasionally  con- 
tains alumina)  gave  no  precipitate.  The  results  showed 
that  the  pumice  had  lost  3.2  per  cent,  of  silica,  the  volcanic 
ash  4.2  per  cent.,  while  the  shale  had  lost  70.3  per  cent. 
The  powdered  residue  from  this  shale  was  subjected  to- 
microscopic  examination,  and  with  the  higher  powers  was 
found  to  contain  a  large  percentage  of  minute  crystalline 
fragments.  Nothing  could  be  made  of  the  isotropic  material 
of  the  residue.  A  considerable  amount  of  the  residue 
thrown  into  dilute  acid  produces  momentary  effervescence, 
showing  that  a  part  of  it  is  calcareous,  doubtless  organic 
remains. 

Origin  of  the  Shale. — These  results  show  that  the  shale  is 
largely  composed  of  opaline  silica,  and,  together  with  the 
statement  of  Dr.  Hinde,  are  sufficient  to  disprove,  for  this 
region  at  least,  the  hypothesis  tentatively  advanced  by  Prof. 
Lawson1  that  the  Miocene  shale  of  the  coast  of  California  is 
largely  of  volcanic  origin.  That  this  shale  is  a  part  of  the 
same  Miocene  shale  which  is  found  so  extensively  developed 
along  the  coast,  there  can  be  little  doubt,  although  the  proof 
obtained  is  not  positive.  It  has  a  similar  appearance,  presents 
the  same  variations,  contains  abundant  micro-organisms  (a 
characteristic  feature  of  the  Miocene  shales),  while  the 
occurrence  of  fish  scales  adds  another  link  to  the  chain  of 
evidence,  as  this  is  another  marked  characteristic  of  the 
coastal  shales.  Further,  the  Miocene  shale  occurs  at  San 

1 "  The  Geology  of  Carmelo  Bay,"  by  Andrew  C.  lyawson.  Bull.  Dept.  Geol.,  Univ. 
Cal.,  Vol.  i,  pp.  24-26. 

(4)  January  n,  1897. 


50  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

Clemente  Island,  some  twenty-five  miles  further  south. 
If,  then,  these  deposits  are  a  part  of  the  extensive  Miocene 
shales,  we  cannot  consider  their  development  as  in  any  way 
local,  or  as  influenced  by  the  deposits  with  which  they  are 
associated,  for  the  conditions  favorable  for  the  development 
of  the  micro-organisms  of  the  shale  at  that  time  must  have 
been  far  reaching.  Except  for  the  few  microscopic  frag- 
ments, the  shale  as  it  occurs  here  is  in  general  peculiarly 
free  from  the  tufaceous  material  with  which  it  is  associated, 
indicating  a  considerable  interval  of  quiet  deposit,  inter- 
rupted at  times  by  violent  local  volcanic  outbursts.  Santa 
Catalina  at  this  time  was  an  island,  as  now,  though  doubtless 
separated  from  the  mainland  by  a  considerably  greater 
expanse  of  water.  The  true  relative  attitude  of  island  and 
mainland  and  the  intermediate  channel  can  be  revealed 
only  from  the  neighboring  coastal  formations.  In  addition 
to  other  evidence,  the  porous  character  of  some  of  the 
lavas  associated  with  the  tuff  and  diatomaceous  earth,  and 
the  remains  of  large  pectens  found  in  the  tuff,  indicate  that 
these  deposits  could  not  have  been  formed  in  abyssal  depths. 
It  would  appear  from  other  reasons  that  they  were  laid  down 
in  not  more  than  600  or  1,000  feet  of  water. 

Analysis  of  Limestone. — For  the  purpose  of  ascertaining 
the  character  of  the  limestone,  a  partial  analysis  was  made 
by  dissolving  fragments  in  acid,  and  determining  the  amount 
of  the  constituents  in  the  solution  thus  obtained.  The 
results  were  as  follows : 

Insoluble  residue 8.234 

Fe22033(alittle)}  2'862 

CaO 27.944 

MgO 13.012 

Ignitionandj   43  6l5 

No  determination  of  the  alkalies  was  made.  A  qualita- 
tive examination  was  made  for  phosphoric  acid,  but  no 
quantitative  determination,  though  considerable  was  found 
to  be  present.  The  residue  in  this  case  also  was  examined 


GEOL.— VOL.  L]         SMITH— SANTA  CAT  A  LIN  A  ISLAND.  51 

microscopically.  While  the  slide  of  the  rock  showed  no 
fragmental  material,  this  powder  showed  a  very  fewiscattered 
mineral  fragments  here  and  there,  but  even  in  this  con- 
centrated form  they  are  not  so  numerous  as  in  the  slides  of 
the  shaly  specimens.  Aside  from  these  the  residue  is  wholly 
isotropic  and  consists  largely  of  the  remains  of  diatoms, 
with  occasional  Radiolaria.  It  is  interesting  to  note,  in  com- 
paring the  chemical  and  microscopical  characters  of  the 
rock,  what  a  small  percentage  of  organic  remains  is  neces- 
sary to  give  a  slide  the  appearance  of  being  well  filled  with 
them. 

The  results  thus  show  that  the  rock  is  a  magnesian  lime- 
stone. Without  doubt  the  carbonates  are  original  and  not 
secondary,  their  source  probably  being  the  sea-water  in 
which  the  organic  forms  were  laid  down. 

C.     SEDIMENTARY  DEPOSITS. 

Besides  the  beds  at  the  isthmus,  two  minor  deposits  of 
sedimentary  material  were  found,  both  within  the  main  area 
of  the  andesite.  One  occurs  in  a  saddle  on  the  principal 
ridge  to  the  west  of  Orizaba,  at  a  distance  of  not  more  than 
a  mile;  the  other,  shown  by  a  dotted  surface  on  the  map,  is 
on  the  lower  slopes  of  the  andesite  in  the  Little  Harbor 
region. 

The  first  mentioned  is  only  a  small  deposit  in  the  lowest 
part  of  the  saddle,  extending  not  more  than  five  or  six  feet 
up  the  slope  on  either  side.  The  rock  which  forms  the 
deposit  proper  is  more  or  less  open  and  is  composed  almost 
wholly  of  coarse  shell  fragments  roughly  cemented  with 
secondary  calcite.  These  fragments,  though  too  small  for 
a  specific  determination,  appear  to  be  the  remains  of  large 
pectens.  In  addition  to  this,  there  were  seen  a  number  of 
rolled  pebbles  of  andesite  and  porphyrite,  and  several  large 
blocks  of  a  white  earthy  material,  all  of  which  appear  to 
have  been  deposited  here.  It  is  possible,  however,  that  they 
were  brought  here  by  the  Indians  who  formerly  occupied 
the  island,  since  there  is  unmistakable  evidence  that  this 


52  CALIFORNIA  ACADEMY  OF  SCIENCES.      [PROC.  30  SER. 

saddle  was  one  of  their  camping  grounds.  The  specimens 
from  the  earthy  blocks  show  a  free  effervescence  with  dilute 
acid,  and  contain  in  places  rough,  free,  calcite  crystals, 
several  millimeters  in  diameter. 

The  lower  slopes  of  the  andesite  in  the  Little  Harbor 
region,  up  to  an  altitude  of  six  or  seven  hundred  feet,  are 
everywhere  strewn  with  rolled  pebbles  of  andesite,  por- 
phyrite,  and  quartzite.  Near  the  northern  border  of  these 
lower  levels  there  are  two  small  deposits  of  white,  earthy 
material.  Along  the  southern  border,  on  the  ridge  adjoin- 
ing Middle  Ranch  Canon,  there  is  a  considerable  deposit  of 
sandstone  and  conglomerate,  and  a  little  above  this  on  the 
same  slope  another  deposit  of  the  earthy  material.  All  the 
specimens  of  the  latter  rock  wherever  found  effervesce 
freely  with  dilute  acid.  A  very  few  rough  shell  casts  were 
found  in  one  of  the  areas  of  the  earthy  rock.  The  rock 
powder  under  the  microscope  showed  no  organic  remains. 
The  bulk  of  the  powder  gives  the  high  polarization  colors 
of  calcite.  Some  of  the  specimens  contain  occasional  small 
pebbles.  In  the  coarser  deposits  there  are  all  gradations, 
from  conglomerate,  with  pebbles  averaging  one-half  an 
inch  in  diameter,  to  a  fine-grained,  yellowish,  micaceous 
sandstone.  None  of  these  effervesce  with  acids.  A  search 
for  fossils  revealed  a  few  indeterminable  shell-casts.  These 
deposits  of  sandstone  and  conglomerate  are  in  general 
thin,  though  at  one  point  they  reach  a  thickness  of  about 
fifteen  feet. 

D.     BRECCIA. 

Beginning  near  the  extreme  southeastern  point  of  the 
island,  and  extending  along  the  coast  to  the  northward,  is  a 
small  area  of  quartzite  breccia.  As  seen  in  the  gulches  it  is, 
in  part  at  least,  bedded,  the  beds  varying  in  the  coarseness 
or  fineness  of  their  material.  The  coarser  beds  contain 
occasional  large  blocks  two  feet  or  more  in  diameter,  and, 
rarely,  reaching  a  length  of  several  yards.  The  material 
composing  this  breccia,  so  far  as  can  be  made  out  with  a 
lens,  is  wholly  quartzite,  except  for  occasional  blocks  and 


GEOL.— VOL.  I.]         SMITH— SANTA  CATALINA  ISLAND.  53 

fragments  of  andesite  seen  near  the  upper  part  of  the  series. 
The  bedding  of  the  breccia  is  seen  on  the  upper  p^art  of  the 
cliffs  at  the  northern  end  of  the  area  as  a  rather  coarse 
banding,  approximately  parallel  to  the  shore-line.  Toward 
the  south  the  banding  becomes  somewhat  irregular  and  is 
lost  to  view  some  time  before  the  extreme  point  is  reached. 
Here  the  breccia  is  seen  at  the  base  of  the  cliffs,  and  so  far 
as  could  be  determined  it  is,  in  part  at  least,  included  in  the 
porphyrite  (see  Plate  III,  fig.  i)  which  occurs  here  on  the 
cliffs  just  above. 

This  porphyrite  is  a  white,  much  weathered  rock,  and  it 
is  possible  that  it  occurs  here  as  a  dike  of  considerable  size, 
or  as  an  intrusive  sheet,  and  does  not  belong  to  the  main 
occurrence  of  this  rock.  At  any  rate  it  is  in  some  respects 
unlike  the  porphyrite  as  it  usually  occurs.  Within  the  area 
just  described  the  porphyrite  outcrops  along  the  shore  at  one 
other  point,  at  least,  where  the  rock  is  to  all  appearances 
like  that  of  the  main  area.  The  breccia  at  the  point  of  the 
island  is  cut  by  a  dike  of  greenish  porphyrite  about  two 
feet  wide,  which  also  cuts  the  white  porphyrite  mentioned 
above. 

About  midway  between  Pebbly  Beach  and  the  extremity 
of  the  island  there  is  a  small  beach  at  the  base  of  the  cliffs, 
which  is  partly  made  up  of  boulders  and  smaller  masses  of 
a  conglomerate  resembling  the  breccia  in  the  material  of 
which  it  is  composed.  This  has  apparently  fallen  from  the 
cliffs  above,  although  no  rounded  material  was  anywhere 
seen  in  place  by  the  writer.  So  far  as  observed,  these 
boulders  are  composed  of  water-worn  metamorphic  pebbles, 
imbedded  in  a  large  amount  of  compact,  greenish  cement. 
This  cement  shows  a  marked  effervescence  with  dilute  acid, 
and  under  the  microscope  it  is  seen  to  be  composed  in  large 
part  of  angular  fragments  of  quartz  and  quartzite,  in  a  thin 
cement  which  is  largely  calcite.  Several  small  sections 
were  seen,  closely  resembling  the  porphyrite  in  appearance, 
and  containing  porphyritic  feldspars,  but  much  altered  by 
decomposition  products. 

The  observations  made  were  too  limited  to  prove  conclu- 


54  CALIFORNIA  ACADEMY  OF  SCIENCES.      LPROC.  30  SER. 

sively  the  relations  of  the  breccia  to  the  main  body  of  the 
porphyrite  and  to  the  andesite,  but  some  of  the  evidence 
points  to  the  probability  of  its  being  older  than  either. 

E.     BASEMENT  SERIES. 

In  surface  area  the  rocks  of  the  basement  series  cover  a 
little  more  than  half  the  island.  They  consist  mainly  of 
quartzites  and  mica -schists,  with  several  smaller  areas  of 
talc-  and  amphibole-schists  and  serpentine. 

The  occurrences  of  the  other  basement  rocks  within  the 
quartzite  area  were  not  exactly  mapped,  and  for  that  reason 
they  appear  on  the  map  without  definite  boundaries.  The 
actinolite  areas,  in  particular,  are  more  extensive  than  is 
here  indicated.  The  main  occurrences  of  these  rocks  are  as 
indicated,  but  smaller  areas  of  all  of  them  occur  elsewhere 
within  the  quartzite  area.  The  actinolite-schists,  besides 
their  main  occurrence,  are  found  in  the  area  of  the  quartzite 
about  Middle  Ranch  Canon.  The  areas  of  the  basement 
rocks  found  within  the  porphyrite  area  northwest  of  Avalon 
are  in  part  of  actinolite-  and  talc-schists,  with  some  ser- 
pentine. Talc-schist  is  also  found  near  the  center  of  the 
west  end,  toward  the  northern  coast.  Besides  the  ser- 
pentine areas  mapped,  a  small  patch  was  found  on  the  ridge 
to  the  south  of  Middle  Ranch  Canon,  and  another  not  far 
from  the  extreme  northwestern  point  of  the  island.  A 
patch  of  garnet  amphibolite  is  found  just  to  the  west  of  the 
border  of  the  andesite  in  the  Little  Harbor  region. 

I.     QUARTZITE. 

The  quartzite  occurs  distinctly  bedded,  and  wherever  it 
was  possible  observations  for  dip  and  strike  were  made, 
though  these  were  insufficient  to  warrant  a  statement  with 
regard  to  the  beds  as  a  whole.  In  many  places  these  read- 
ings give  no  real  indication  of  the  general  dip  and  strike  of 
the  series,  owing  to  local  folding  with  minor  plications,  and 
to  occasional  faulting.  By  a  comparison  of  the  various 
readings  made,  however,  the  western  division  of  the  island 


GEOL.— VOL.  I.]        SMITH—SANTA  CATALINA  ISLAND.  55 

appears  to  have  a  synclinal  structure.  Whether  this  is  true 
for  the  division  to  the  southeast  of  the  isthmus_  cannot  be 
stated. 

An  excellent  cliff  section  showing  the  general  stratifica- 
tion is  seen  on  the  southern  coast  of  the  west  end,  where 
the  bedding  is  distinctly  visible  for  three  or  four  miles  along 
the  shore.  For  the  greater  part  of  the  distance  the  dip  of 
the  beds  is  quite  uniform,  though  the  minor  beds  and  sheets 
observed  show  intricate  folding  and  crumpling.  The  dip 
ranges  from  S.  15°  E.  to  S.  45°  E.,  at  an  angle  varying 
from  15°  to  about  30°. 

On  the  northern  coast  of  this  part  of  the  island  the  dip, 
so  far  as  observed,  is  northerly,  and  varies  considerably  in 
amount,  the  average  lying  between  25°  and  50°.  A  char- 
acteristic section  of  the  bedding  is  shown  in  Plate  III,  fig.  i, 
a  view  of  the  shore  at  the  north  end  of  the  beach  of  Cherry 
Valley,  the  second  small  bay  to  the  north  of  Isthmus  Cove. 

The  quartzites  are  nearly  everywhere  intersected  by  nu- 
merous veins  of  secondary  quartz,  usually  of  small  size  and 
running  in  various  directions.  In  places,  however,  these 
veins  attain  a  width  of  a  foot  or  more.  At  a  number  of 
points  some  of  the  veins  contain  a  small  percentage  of  min- 
eral ores. 

The  quartzite  is  usually  bedded  in  thin  and  more  or  less 
irregular  sheets.  They  range  from  a  fraction  of  an  inch  to 
two  or  three  inches  in  thickness,  averaging  perhaps  half  an 
inch.  These  sheets  are  usually  separated  by  partings  of  a 
dark  earthy  character,  varying  in  thickness  from  the  thin- 
nest film  to  about  a  quarter  of  an  inch.  In  the  more  thinly 
bedded  quartzite  these  partings  are  frequently  thicker  than 
the  quartzite  sheets. 

Macroscopic  Characters. — The  quartzite  is  occasionally 
milky  white;  usually,  however,  as  seen  with  a  lens,  it  ap- 
pears colorless  and  glassy.  Rarely  it  is  found  black,  while 
here  and  there  it  occurs  with  a  tinge  of  pink,  or  even  con- 
siderably reddened,  owing  to  the  presence  of  minute  gar- 
nets, either  scattered  through  the  sheet  or  arranged  in  bands 


56  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  3D  SER. 

parallel  to  the  bedding.  Many  of  these  quartzite  sheets 
appear  to  be  wholly  free  from  mica,  the  surface  of  the  sheet 
glistening  from  the  minute  quartz  crystals  composing  the 
rock.  Other  specimens,  in  cross-section,  appear  to  be  of 
clear  quartz,  but  when  viewed  in  the  plane  of  the  bedding 
numerous  minute  scales  of  muscovite  are  seen  scattered 
over  the  surface.  In  all  fractured  surfaces  these  flakes  are 
seen  to  be  arranged  with  their  planes  parallel  to  the  plane 
of  the  bedding.  There  are  all  gradations  between  this  and 
specimens  in  which  the  mica  is  the  most  prominent  mineral. 

The  layers  which  form  the  partings  of  the  quartzite  beds 
are  quite  dark,  varying  from  a  dark  gray  to  a  yellowish  or 
reddish  color  due  to  iron  stain.  They  are  finely  schistose 
and  readily  flake  off;  are  quite  soft  and  have  usually  a 
smooth,  silvery  surface.  Even  where  this  silvery  luster  is 
not  at  first  apparent  it  may  easily  be  made  out  with  a  lens. 
The  layers  appear  to  be  composed  of  mica  or  its  decompo- 
sition products.  Tested  chemically  the  mineral  shows  the 
presence  of  a  large  amount  of  alumina,  a  little  iron,  no  lime 
and  a  little  magnesia,  besides  giving  a  decided  flame  reac- 
tion for  potassium.  The  optical  characters  of  the  flakes 
could  not  be  determined,  owing  to  their  want  of  transpar- 
ency. 

A  considerable  proportion  of  the  rocks  of  the  west  end 
have  much  the  appearance  of  gray  sandstone  to  the  unaided 
eye,  though  with  a  lens  they  are  seen  to  be  composed 
largely  of  this  micaceous  material,  with  minute  lenses  or 
grains  of  the  quartzite  scattered  through  it.  These  mica- 
schists  occur  indiscriminately  with  the  rocks  which  are 
more  properly  quartzites,  and  occasionally  lens -shaped 
masses  of  the  quartzite  are  found  in  such  areas.  There 
are  all  gradations  between  these  rocks  in  which  the  mica  is 
predominant  and  those  in  which  the  quartz  predominates. 

Besides  the  micaceous  partings  of  the  quartzites  there 
were  found  at  a  number  of  points  partings  of  blue  amphi- 
bole,  having  frequently  a  silky  luster.  This  amphibole  also 
occurs  in  larger  masses  in  a  schistose  condition.  The  oc- 
currences of  this  rock  were  not  mapped,  but  they  are  found 


GEOL.— VOL.  I.]        SMITH— SANTA  CAT  A  LIN  A  ISLAND.  57 

particularly  in  the  Little  Harbor  region,  apparently  confined 
to  the  neighborhood  of  the  areas  of  the  amphibole-  and  talc- 
schists  and  serpentine.  It  is  probable  that  here,  as  else- 
where in  California,  these  blue  amphibole-schists  are  due  to 
local  contact  metamorphism  occasioned  by  the  intrusion  of 
basic  irruptives.1 

Microscopic  Characters. — Only  one  slide  was  made  of 
the  quartzites,  which,  however,  is  doubtless  typical  of  the 
purer  quartzites  in  general.  It  consists  almost  entirely 
of  a  mosaic  of  clear  quartz  grains  of  irregular  shape  and 
size.  Many  of  them  are  flattened  in  a  direction  parallel  to 
the  plane  of  schistosity,  thus  giving  frequently  very  much 
elongated  sections.  Their  boundaries  interlock  in  an  ex- 
tremely intricate  manner.  Occasional  pale  pink  garnets 
occur  as  inclusions  in  the  quartz,  averaging  a  little  less  than 
.1  mm.  in  diameter.  They  are  for  the  most  part  rounded, 
though  two  or  three  present  crystal  boundaries.  Long  nar- 
row sections  of  what  is  probably  sillimanite  are  compara- 
tively numerous,  nearly  all  arranged  with  their  longer  axes 
parallel  to  the  plane  of  schistosity  or  to  the  direction  of  the 
flattening  of  the  quartz  grains.  The  terminals  taper  more 
or  less  gradually  to  a  point.  No  cross -sections  were  seen. 
The  mineral  is  colorless  and  has  a  moderately  high  index 
of  refraction,  somewhat  higher  than  that  of  quartz,  and  it 
may  therefore  be  readily  distinguished  from  the  latter  in 
ordinary  light.  The  double  refraction  is  considerable,  giv- 
ing brilliant,  though  somewhat  mottled,  polarization  colors. 
The  extinction  is  in  all  cases  parallel  and  perpendicular  to 
the  longer  axis  of  the  mineral.  In  all  the  sections  observed 
the  longer  axis  is  the  axis  of  less  elasticity  as  shown  by  the 
quartz  wedge.  No  optical  figure  was  obtained. 

1 "  A  Contribution  to  the  Geology  of  the  Coast  Ranges,"  by  Andrew  C.  Lawson.  Am. 
Geol.,  Vol.  XV  (June,  1895),  p.  352. 

"  The  Geology  of  Angel  Island,"  by  F.  Leslie  Raiisome.  Bull.  Dept.  Geol.,  Univ.  Cal., 
Vol.  i,  No  7. 


58  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  3D  SER. 

2.       ACTINOLITE    AND    HORNBLENDE    SCHISTS. 

The  actinolite-schist  occurs  bedded,  showing  greater 
variation  than  the  quartzite  in  the  thickness  of  the  beds.  It 
also  frequently  exhibits  plications  such  as  occur  in  the 
quartzite.  Some  of  these  schists  occur  in  rather  thin 
beds,  with  a  finely  schistose  structure,  the  slender  needles 
of  actinolite  parallel  to  the  plane  of  schistosity.  In  other 
cases,  especially  the  coarser  forms  of  the  rock,  it  is 
found  showing  no  marked  schistose  arrangement.  The 
crystals  in  these  coarser  schists  frequently  have  a  length  of 
three  or  four  centimeters.  The  rocks  are  more  or  less 
compact,  and  in  general  are  composed  of  columnar  or  aci- 
cular  actinolite  crystals,  but  always  associated  with  a  greater 
or  less  amount  of  other  minerals.  The  most  common 
mineral  accompanying  the  actinolite  is  talc.  This  is  usually 
in  small  amounts,  but  rarely  it  becomes  the  dominant  min- 
eral, forming  a  matrix  in  which  the  needles  of  actinolite  are 
embedded.  Chlorite  occasionally  occurs  with  the  actinolite, 
and  like  the  talc,  this  sometimes,  though  rarely,  becomes 
dominant.  The  chlorite  varies  in  occurrence  from  minute 
flakes  to  plates  several  centimeters  in  diameter.  A  small 
amount  of  quartz  is  frequently  found  in  these  schists,  and 
occasionally  both  quartz  and  feldspar,  in  varying  amounts, 
occur  associated  with  the  actinolite  and  hornblende. 

Almost  the  entire  area  of  actinolite-  and  hornblende-schist 
is  composed  of  the  former.  The  latter  is  confined  to  the 
area  which  contains  the  serpentine,  occurring  here  with  the 
actinolite-schist.  The  rocks  are  coarse-grained,  compact, 
greenish  black  in  color,  and  are  composed  of  coarsely  pris- 
matic crystalline  hornblende.  A  small  amount  of  mica  is 
occasionally  associated  with  it. 

3.     SERPENTINE. 

The  serpentine  of  this  same  area  is  found  on  the  summits 
of  these  hills  of  amphibole-schist.  The  hills  are  in  the 
neighborhood  of  1,000  feet  in  height,  and  the  serpentine 
which  outcrops  here  is  two  or  three  hundred  feet  in  thick- 


GEOL.— VOL.  I.]        SMITH— SANTA   C ATA  LIN  A  ISLAND.  59 

ness.  It  occurs  stratiform,  with  an  average  dip  of  from  20° 
to  30°  in  a  northerly  direction.  The  rocks  are  very  hard  and 
compact,  and  in  weathering  present  an  extremely  rough 
surface,  with  projecting  fragments,  many  of  which  have 
sharp,  jagged  points.  It  is  doubtless  owing  to  this  bold, 
irregular  surface  that  one  of  these  hills  has  received  the 
name  of  Granite  Peak.  The  surface  of  this  rock  is  also 
irregularly  pitted.  The  occurences  are  almost  wholly  of 
this  facies,  and  little  evidence  was  seen  of  internal  move- 
ment, causing  a  slickensided  appearance.  The  general 
appearance  of  the  rock  in  the  field  is  in  most  respects  quite 
unlike  that  of  the  serpentine  of  the  Potrero,  San  Francisco, 
described  by  Dr.  Palache,1  which  is  typical  of  much  of  the 
serpentine  of  the  Coast  Range.  There  are  a  few  small 
patches  of  magnesite  within  the  serpentine  area.  The  hand- 
specimens  of  the  serpentine  vary  in  color  from  a  dirty  greenish 
white  to  a  dark  bluish  green,  more  or  less  mottled  with 
limonite.  The  compact  specimens  show  an  indistinctly 
banded  structure,  and  have  a  rather  uneven  fracture.  This 
surface  is  entirely  different  from  the  smooth  and  somewhat 
polished  surface  of  the  pale  green,  slickensided  specimens. 
Traversing  the  surface  in  lines  approximately  parallel  to  the 
banding  are  occasional  fine  veins  and  threads  of  chrysotile, 
with  their  fibres  at  right  angles  to  the  enclosing  walls,  and 
stained  here  and  there  with  iron.  More  numerous  and  finer 
threads  cross  the  surface  at  right  angles  to  the  larger  veins, 
and  nearly  all  are  stained  with  limonite.  Threads  of  mag- 
netite run  through  the  rock,  in  no  fixed  direction.  In  some 
places  the  rocks  contain  many  minute  veins  of  secondary 
silica,  running  at  right  angles  to  the  banding.  Cross-sections 
seen  on  the  surface  show  that  they  are  filled  with  the  silica 
arranged  in  concentric  rings.  No  remnants  of  the  minerals 
from  which  the  serpentine  was  derived  were  seen  in  any  of 
the  specimens,  but  it  doubtless  consisted  in  large  part  of 
olivine,  for  the  mesh-structure  characteristic  of  the  serpen- 

^'The  Lherzolite-Serpentine  and  Associated  Rocks  of  the  Potrero,  San  Francisco." 
Bull.  Dept.  Geol.,  Univ.  Cal.,  Vol.  i,  No.  5,  pp.  161-179. 


60  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

tines  so  derived  is  seen  throughout  the  greater  part  of  the 
slides. 

Areas  of  a  somewhat  different  fades  of  the  serpentine 
occur  within  the  talc  and  garnet-amphibolite  area.  This 
rock  is  hard,  compact,  occurring  massive,  and  not  strati- 
form. As  in  the  serpentines  just  described,  minute  veins 
of  silky  chrysotile  traverse  it  here  and  there.  The  rock  is 
very  dark  green,  and  scattered  through  it  are  aggregates  of 
a  magnesian  mineral,  with  pearly  luster,  whose  optical  prop- 
erties were  not  investigated.  Besides  this  mineral,  there 
occurs  in  various  amounts,  associated  with  the  compact  ser- 
pentine, a  pale  green,  lamellar  mineral  with  the  optical 
properties  of  bastite. 

4.     TALC-SCHIST. 

The  area  in  which  this  serpentine  is  found  is  largely  of 
talc-schist,  usually  found  as  a  soft,  foliated  rock,  stained 
yellow  with  limonite.  It  has  a  silvery  luster,  and  when 
looked  at  closely  the  talc  is  seen  to  be  of  a  pale  green  color. 
It  is  quite  smooth,  with  a  greasy  feel,  and  is  easily  scratched 
with  the  nail.  The  rock  splits  readily  along  the  schistose 
surfaces.  Near  the  western  end  of  the  area,  back  of 
Empire  Landing,  there  is  a  soapstone  quarry  where  is  found 
a  facies  of  the  schist,  which  but  little  resembles  the  foliated 
form  just  described.  This  is  massive,  not  schistose,  and 
has  a  dark  gray  color  with  a  tinge  of  green.  The  rock 
may  be  scratched  with  the  nail  only  in  places,  showing  that 
it  is  not  wholly  talc.  The  chief  difference  between  this 
rock  and  the  foliated  schist  is  the  presence  everywhere 
through  it  of  a  mineral  with  a  pronounced  lamellar  struc- 
ture, occurring  in  moderately  small,  bladed  forms,  which 
are  interlaced  in  all  directions.  This  mineral  appears  to  be 
the  chief  constituent  of  the  rock,  and  at  least  equal  to  the  talc 
in  amount.  It  is  pale  green  in  color,  with  a  metalloidal  luster, 
and  a  hardness  of  about  4.  Before  the  blowpipe  it  gives 
the  characteristic  reactions  of  serpentine.  With  a  lens  the 
silvery  flakes  of  talc  may  be  seen  here  and  there,  besides 
scattered  grains  of  pyrite. 


GEOL.— VOL.  I.]        SMITH— SANTA  C ATA  LIN  A  ISLAND.  6 1 

Under  the  microscope  the  rock  is  seen  to  be  composed  in 
part  of  an  allotriomorphic  aggregate  of  bastite,  and  partly 
of  irregular  areas  of  talc,  with  several  small  patches  of 
magnesite.  Small  amounts  of  pyrite  are  scattered  through 
the  slide. 

The  bastite  occurs  in  plates  or  somewhat  lath-shaped  forms, 
and  is  colorless  or  with  the  faintest  tinge  of  green.  The 
mineral  is  non-pleochroic,  even  in  moderately  thick  cleavage 
flakes.  It  has  a  pronounced  fibration  parallel  to  the  vertical 
axis.  Its  extinction  is  characteristic  of  a  rhombic  mineral, 
being  in  all  cases  parallel  to  this  fibration.  It  has  a  low 
index  of  refraction,  and  gives  low  interference  colors,  much 
like  those  of  feldspar.  The  cleavage  flakes  show  the  fibra- 
tion which  is  observed  in  thin  section.  Rarely  a  needle  of 
pyrite  is  seen  in  the  fibration.  Cleavage  flakes  give  a  good 
biaxial  interference  figure,  and  show  that  the  plane  of  the 
optical  axes  is  at  right  angles  to  the  plane  of  cleavage  and 
parallel  to  the  fibration.  The  optical  character  of  the  min- 
eral is  negative,  as  determined  both  by  the  mica  plate  and 
quartz  wedge.  The  bastite  is  everywhere  altering  to  talc, 
and  all  stages  of  the  process  may  be  seen.  Alteration  begins 
along  the  margin  and  along  the  cleavage  planes,  and  works 
inward.  Occasionally  the  talc  occurs  as  a  pseudomorph 
after  the  bastite,  giving  a  parallel  extinction,  owing  to  a  par- 
allel arrangement  of  the  fibres  of  the  talc.  Usually,  how- 
ever, the  talc  occurs  in  patches  of  irregular  shape,  and  with- 
out a  definite  extinction  throughout  an  entire  revolution  of 
the  stage,  owing  to  the  compensatory  effect  of  the  irregu- 
larly oriented  talc  fibres. 

5.     ORIGIN  OF  THE  SERPENTINES. 

No  detailed  petrographical  study  was  made  of  the  serpen- 
tine rocks  of  the  island,  but  such  as  was  made  proves  them 
to  be  variable  in  their  microscopic  structure,  and  therefore 
different  in  their  origin.  At  no  point  was  there  seen  any  of 
the  unaltered  rock  from  which  the  serpentine  was  derived, 
so  that  the  conclusions  must  be  drawn  from  the  microscopic 


62  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

structure  of  the  serpentine  itself.  Judging  from  this,  the 
serpentines  may  be  roughly  divided  into  three  groups  accord- 
ing to  their  probable  origin:  (i)  those  derived  from  pyrox- 
enites,  (2)  those  from  rocks  composed  largely  of  olivine, 
and  (3)  those  from  a  rock  in  which  both  rhombic  pyroxene 
and  olivine  were  among  the  essential  constituents.  The 
first  are  now  characterized  by  the  bastite  structure,  the 
second  by  the  mesh-structure.  It  is  probable  that  the  whole 
of  the  talc-schist  is  derived  from  the  first  form  of  serpen- 
tine. 

6.      GARNET-AMPHIBOLITE. 

Along  the  ridge  near  the  upper  limit  of  the  talc-schists, 
and  within  that  area,  are  found  here  and  there  small,  pro- 
jecting bosses,  with  occasional  larger  areas,  of  garnet-am- 
phibolite.  This  rock  usually  presents  a  somewhat  roughened 
surface,  more  or  less  reddened  with  iron  oxide.  It  is  not 
compact,  and  readily  crumbles  under  the  hammer.  The 
fresher  material  is  dark  or  almost  black  in  color,  and  appears 
to  be  composed  wholly  of  a  brownish  or  greenish  hornblende, 
with  roughly  rounded  red  garnets  in  varying  size  and 
amount.  In  some  places  these  garnets  attain  a  diameter  of 
about  3  mm.  and  form  the  principal  feature  of  the  rock, 
while  in  other  cases  the  rock  is  composed  almost  entirely  of 
a  somewhat  fine-grained  hornblende,  and  an  occasional 
minute  garnet  may  be  made  out  only  with  the  aid  of  a  lens. 
At  a  few  points  the  rock  occurs  as  a  black,  rather  coarsely 
granular  aggregate,  composed  entirely  of  hornblende,  so  far 
as  can  be  determined  with  a  lens. 

A  slide  was  made  of  the  facies  of  the  rock  containing 
the  largest  garnets.  There  are  nine  of  these  garnets  in  the 
slide,  ranging  from  2  to  3  mm.  in  diameter.  Microscopically 
the  rock  is  composed  of  scattered,  pale  pink  garnets  in  a 
matrix  of  hornblende.  Here  and  there  are  small  grains  of 
rutile.  The  hornblende  is  brownish  with  a  tinge  of  green, 
and  occurs  in  allotriomorphic  plates,  with  seldom  a  hint  of 
crystallographic  form.  The  boundaries  are  usually  well 
marked  by  a  limonite  stain.  The  sections  themselves  are 


GEOL.— VOL.  I.]        SMITH— SANTA  CATALINA  ISLAND.  63 

quite  fresh  and  free  from  products  of  decomposition.  The 
mineral  has  a  pronounced  prismatic  cleavage.  _It  is  quite 
strongly  pleochroic,  c  being  dark,  greenish  brown,  6  deep, 
yellowish  brown,  and  a  very  pale,  brownish  green.  The 
absorption  scheme  is  c^l6>a.  Inclusions  are  common,  and 
in  many  of  the  sections  abundant.  They  are  largely  minute 
flakes  of  a  mineral  with  low  polarization  colors,  and  a 
refractive  index  somewhat  higher  than  that  of  the  horn- 
blende. The  same  mineral  occurs  in  scattered  flakes  in  the 
garnets  also,  and  they  are  there  seen  to  be  colorless  or 
nearly  so.  In  the  hornblende  these  flakes  are  in  small, 
open  areas,  usually  collected  near  the  center  of  the  includ- 
ing crystal.  Besides  these  inclusions,  occasional  small 
grains  of  rutile  are  found. 

The  rutile,  in  general,  through  the  slide,  occurs  as  rounded 
and  usually  oblong  grains,  in  color  deep  yellowish  to  red- 
dish brown,  varying  with  the  tints  of  amber.  These  grains 
are  usually  found  along  the  lines  which  mark  the  bounda- 
ries between  the  hornblendes,  and  generally  several  together 
occur  along  the  same  line.  They  have  an  extremely  high 
index  of  refraction,  and  on  account  of  the  consequent  dif- 
fusion of  light  the  extinctions  are  not  sharp  and  clear.  The 
direction  of  extinction  in  the  grains  which  are  distinctly 
elongated  is  parallel  and  at  right  angles  to  the  axis  of  elon- 
gation. The  mineral  shows  a  pronounced  though  not 
strong  pleochroism,  and  has  a  strong  absorption.  As  the 
grains  could  not  be  distinguished  from  the  dark  hornblende 
in  the  crushed  rock,  the  hornblende  was  dissolved  out  by 
means  of  hydrofluoric  acid,  when  the  minute,  dark  grains 
of  the  rutile  could  be  readily  distinguished  from  the  pale 
red  fragments  of  garnet,  neither  of  these  being  attacked  by 
the  acid.  Some  of  these  grains  were  then  separated  and 
tested  for  titanium,  with  favorable  results. 

The  garnets  have  quite  irregular  boundaries,  and  along 
the  margin  are  frequently  intergrown  with  the  hornblendes 
which  surround  them.  In  a  number  of  cases  minute  frag- 
ments of  the  garnets  are  completely  enclosed  by  the  border- 
ing hornblendes,  while  occasionally  a  fragment  of  horn- 


64  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

blende  is  wholly  surrounded  by  the  garnet  near  its  margin. 
Frequent  cracks,  many  of  them  iron -stained,  intersect  the 
garnets,  without  definite  direction.  Macroscopically  the 
garnets  appear  to  have  a  zonal  structure,  with  a  narrow  and 
somewhat  clouded  outer  zone,  a  broad  middle  zone,  seem- 
ingly of  the  clear  pink  garnet,  and  a  slightly  darker  inner 
zone.  Under  the  microscope  this  structure  is  seen  to  be 
due  to  inclusions  in  the  garnet.  With  crossed  nicols  the 
sections  are,  as  a  whole,  not  perfectly  isotropic,  but  trans- 
mit a  faint  light  in  all  positions.  This  is  due  to  a  multitude 
of  microscopic,  dust-like  inclusions,  which  fill  the  central 
portions  of  the  garnets.  With  higher  powers  these  inclu- 
sions cannot  be  resolved,  but  are  seen  to  be  of  some  rather 
brightly  polarizing  mineral.  They  do  not  always  occur  in 
solid  areas,  for  portions  of  the  space  are  free  from  them, 
these  isotropic  portions  running  like  veins  through  the  mass. 
These  areas  fill  the  greater  part  of  the  sections,  but  there  is 
always  a  narrow,  irregular  band  along  the  margin  which  is 
free  from  these  minute  inclusions,  and  is  isotropic  under 
crossed  nicols.  The  darker,  central  areas  appear  to  be  due 
to  a  clouding,  the  nature  of  which  could  not  be  determined. 
Besides  these  minute  inclusions  there  are  others  scattered 
through  the  slide,  which  have  been  mentioned  in  connection 
with  the  hornblende.  Some  of  the  garnets  contain  here  and 
there,  particularly  along  the  isotropic  borders,  minute  needles 
of  a  yellowish  to  brownish  mineral,  with  parallel  extinction 
and  high  refractive  index,  and  giving  high  polarization  col- 
ors. This  mineral  is  probably  rutile.  A  few  of  these  nee- 
dles were  seen  in  some  of  the  hornblendes  bordering  the 
garnets  in  which  the  inclusions  are  found,  and  some  of  the 
needles  were  seen  extending  from  the  one  mineral  into  the 
other.  There  are  rarely  inclusions  of  large  grains  of  rutile 
and  small  hornblendes. 


GEOL.— VOL.  I.]        SMITH— SANTA   CAT  A  LIN  A  ISLAND.  65 

IV.     GEOMORPHOGENY. 

i.     SUBMARINE  TOPOGRAPHY. 

The  submarine  contours  surrounding  the  island  have  been 
represented  for  depths  of  200,  300,  400  and  600  feet.  The 
discussion  of  the  results  arrived  at  by  a  study  of  this  feature 
of  the  topography  has  been  left  till  this  point,  as  these  re- 
sults are  so  closely  connected  with  the  geological  history  of 
the  island.  By  mapping  in  the  deeper  contours,  it  is  seen 
that  the  general  form  of  the  island  is  preserved  to  a  depth 
of  at  least  1,800  feet,  and  doubtless  somewhat  beyond  this, 
though  the  indications  are  that  the  pronounced  trench  out- 
side Little  Harbor  gradually  loses  its  character,  so  that  at 
some  greater  depth  the  outline  of  the  entire  mass  may  be 
much  simpler. 

In  looking  at  the  map  it  will  be  noticed  that  the  average 
distance  from  the  shore  to  the  200  feet  contour  is  much  less 
than  the  average  distance  from  the  200  feet  to  the  400  feet 
contour.  This  is  particularly  marked  in  those  parts  of  the 
island  where  the  cliff  cutting  is  the  most  rapid.  By  map- 
ping in  the  contours  on  the  large  Coast  Survey  map  of  the 
isthmus  these  features  are  strikingly  brought  out.  Here, 
since  there  is  more  detail,  it  is  readily  seen  that  the  more 
rapid  deepening  of  the  water  near  the  shore  extends  to  about 
250  feet,  and  to  this  level  the  details  of  the  present  outline 
are  fairly  well  preserved.  Beyond  the  250  feet  contour 
there  is  a  broad  platform  with  a  very  gentle  outward  slope 
(of  about  i°)  to  some  point  beyond  the  300  feet  level. 

Beyond  the  400  feet  contour  the  water  deepens  rapidly 
on  the  southern  side  of  the  island,  while  on  the  north  the 
widely  separated  contours  indicate  a  gradual  slope.  The 
pronounced  difference  between  the  two  sides  is  well  shown 
in  the  accompanying  sections  (figs.  5  and  6) ,  which  were 
chosen  as  most  fairly  representing  the  average  character  of 
the  two  sides  respectively.  The  first  is  the  section  along 
a  line  at  right  angles  to  the  outermost  point  north  of  Whit- 
ley's  Cove;  the  other,  along  a  line  at  right  angles  to  the 

(5)  January  12,  1897. 


66 


CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 


shore  on  the  opposite  side  of  the  island,  at  a  point  to  the 
southwest  of  the  most  southern  occurrence  of  the  andesite 
on  the  map.  These  sections  suggest  the  possible  origin  of 
the  island  as  a  tilted  orographic  block,  the  rapid  descent  on 
the  southern  side  contrasting  strongly  with  the  moderate 
slope  on  the  other.  The  contrast  is  similar  to  that  of  sec- 
tions taken  on  opposite  sides  of  San  Clemente  Island  which 
is  almost  certainly  such  a  block.1  The  platform  mentioned 


FIGURE  5 — Submarine  profile,  north  side  of  Santa  Catalina. 


FIGURE  6— Submarine  profile,  south  side  of  Santa  Catalina. 

as  occurring  above  the  350  feet  contour  is  well  brought  out  in 
the  section  from  the  southern  side,  though  the  slight  increase 
in  slope  above  the  200  feet  contour  is  not  marked.  The 
features  make  it  clear  that  before  the  present  sinking  of 
the  island  began  it  stood  some  350  feet  higher  than  now. 
That  this  platform  is  later  than  the  andesite  is  seen  from  the 
fact  that  it  has  been  cut  in  the  tufaceous  deposits  at  the 
isthmus.  The  island  stood  for  some  time  at  or  near  that 
level,  while  rapid  cutting  was  going  on  both  along  the  cliffs 
on  the  most  exposed  sides,  and  in  the  softer  tufaceous 
deposits  near  the  isthmus,  where  the  erosion  of  the  harder 
rocks  is  comparatively  slow.  When  the  island  had  been 


1  "The  Post-Pliocene  Diastrophism  of  the  Coast  of  Southern  California,"  by  Andrew 
C.  Lawson.     Bull.  Dept.  Geol.,  Univ.  Cal.,  Vol.  i,  No.  4,  p.  129. 


GEOL.— VOL.  L]         SMITH— SANTA  CATALINA  ISLAND.  67 

reduced  to  the  form  approximately  shown  by  the  200  feet 
contour  the  present  sinking  began.  This  is_  shown  not 
only  by  the  increase  of  the  submarine  slope,  but  also  in  the 
isthmus  chart,  by  the  preservation  of  the  main  features  of 
the  present  drainage  system,  showing  that  the  recent  stream- 
valley  flooding  took  place  at  that  level.  The  information 
on  this  point  obtained  from  this  latter  source,  however,  is 
reliable  only  within  certain  limits,  as  a  rapid,  partial  or  com- 
plete filling  up  of  these  sunken  channels  in  their  lower 
levels  might  easily  cause  an  error  in  the  interpretation  of 
the  facts.  But,  taken  in  connection  with  the  other  evi- 
dence, they  may  be  considered  as  trustworthy  to  a  certain 
extent  at  least. 

2.     OUTLINE  OF    HISTORY. 

The  alteration  and  deformation  of  the  basement  rocks  of 
Santa  Catalina  probably  took  place  before  the  individualiza- 
tion  of  the  mass  now  forming  the  island.  In  the  opinion  of 
the  writer,  the  history  of  Santa  Catalina  began  with  the 
tilting  of  an  orographic  block  formed  of  the  already  altered 
basement  rocks.  This  view  is  based  on  the  character  of 
the  submarine  contours  and  the  slight  recent  tilting  shown 
in  the  slope  of  the  summits  (see  figs.  2  and  3,  page  7). 
The  tilting  was  no  doubt  gradual,  and  has  continued  inter- 
mittently to  comparatively  recent  times.  This  is  shown  in 
connection  with  the  figures  just  referred  to,  as  it  was  pointed 
out  that  the  angle  of  the  slope  of  the  crest  is  about  i°  from 
the  horizontal,  and  in  the  direction  of  tilting  toward  the 
north.  No  further  evidence  of  the  original  crust-block  is 
seen  on  the  land,  owing  to  the  extensive  erosion  to  which 
the  mass  has  since  been  subjected. 

The  time  of  the  original  tilting  is  not  known,  but  the 
crust-block  must  have  been  at  that  time  a  part  of  the  main- 
land. By  long  continued  erosion  the  crest  of  the  mass  was 
carried  northward  so  that  it  occupied  a  position  now  approx- 
imately represented  by  the  main  ridge  from  Whitley's  Cove 
to  the  west  end.  Following  this  came  the  irruption  of  the 


68  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

porphyrite  laccolite,  possibly  preceded  by  a  further  tilting 
of  the  block.  This  irruption  led  to  the  formation  of  a  struc- 
tural valley  in  the  Little  Harbor  region,  between  the  por- 
phyrite area  and  the  ridge  just  mentioned.  This  valley  was 
subsequently  enlarged  and  deepened  by  an  extensive  ero- 
sion which  followed.  At  this  time  Catalina  probably  stood 
some  two  or  three  thousand  feet  higher  than  now.  The 
mass  then  had  the  general  form  of  two  long  ridges,  the  one 
already  referred  to,  and  another  having  the  general  trend  of 
the  porphyrite  area  as  seen  on  the  map,  and  being  possibly 
connected  with  the  former  ridge  not  far  from  its  eastern 
end.  The  drainage  of  the  large  valley  just  mentioned  was 
to  the  west.  Its  remnants  still  exist  on  the  island,  forming 
the  amphitheater  of  the  Little  Harbor  region. 

This  period  of  erosion  was  followed  by  the  eruption  at 
intervals  of  andesite,  which  completely  filled  a  portion  of 
this  valley  and  covered  the  adjacent  ridges.  The  source  of 
these  outpours  appears  to  have  been  local.  They  were 
accompanied  by  a  slow  settling  of  the  land  area  to  which 
this  mass  then  belonged,  and  Santa  Catalina  became  an  island, 
probably  for  the  first  time  in  its  history.  The  evidence 
shows  that  it  has  remained  an  island  ever  since.  That  it 
was  sinking  at  this  time  is  shown  by  the  deposits  of  tuff 
intercalated  with  the  lavas. 

This  submergence  continued  after  the  andesite  flows  had 
ceased,  for  the  higher  portions  of  the  andesite  were  some- 
what eroded  before  the  island  had  reached  its  lowest  level, 
as  is  shown  by  the  fact  that  the  shelly  deposit  near  Orizaba 
(see  page  51)  lies  in  a  saddle  several  hundred  feet  below 
the  peaks  bordering  it  on  either  side.  The  amount  of  this 
depression  was  between  1,400  and  1,600  feet  below  the  level 
at  which  the  island  now  stands.  That  it  was  at  least  as 
great  as  this  is  shown  by  this  same  shelly  deposit,  which 
occurs  at  an  elevation  of  about  1,360  feet;  and  that  it  was 
not  greater  is  shown  by  the  base-levelled  summits  of  the 
island  at  an  elevation  of  from  1,400  to  1,600  feet.  This 
took  place  during  Miocene  times,  as  the  deposits  of  shale 
near  the  isthmus  bear  witness.  This  submergence  may 


GEOL.— VOL.  I.]         SMITH— SANTA  CAT  A  LIN  A  ISLAND.  69 

have  been  sufficient  to  form  two  islands  of  the  mass,  the 
channel  between  them  extending  from  a  little  to  the  west  of 
the  isthmus,  three  or  four  miles  to  the  east.  This  is  based 
on  the  fact  that  between  these  points  the  main  ridge  falls 
considerably  below  the  1,400  ft.  level.  It  may  be,  however, 
that  this  decrease  in  altitude  is  a  part  of  the  local  depression 
hereafter  suggested  in  connection  with  the  isthmus. 

The  submergence  was  followed  by  a  long  period  of  ero- 
sion, during  which  the  then  existing  island  (or  islands)  was 
reduced  to  the  peneplain  condition.  The  main  body  was  a 
low  and  nearly  level  area,  above  which,  near  the  center, 
projected  the  higher  andesitic  peaks.  This  area  contained 
a  bay  of  considerable  size,  occupying  the  Little  Harbor 
region.  The  reduction  of  the  island  to  a  peneplain  was 
followed  by  an  elevation,  the  amount  of  which  is  approxi- 
mately indicated  by  the  350  feet  submarine  contour,  thus 
making  the  altitude  of  the  peneplain,  roughly,  1,850 
feet.  This  movement  was  gradual,  and  was  interrupted 
by  at  least  one  pause,  at  an  elevation  of  600  or  700  feet 
above  the  present  sea-level.  This  is  shown  by  the  levelled 
slopes  in  the  lower  portion  of  the  Little  Harbor  region,  and 
by  the  sedimentary  deposits  found  on  these  slopes.  The 
island  remained  at  its  highest  level  long  enough  to  carve  the 
broad  submarine  bench  on  the  most  exposed  side.  A  very 
slow  subsidence  may  have  taken  place  at  this  time.  It  was 
followed  by  the  present  period  of  comparatively  rapid  sink- 
ing. 

This  most  recent  period  has  been  a  short  one,  as  is  shown 
by  the  small  amount  of  cliff  cutting,  which  has  taken  place 
since  it  began,  on  those  parts  of  the  island  most  exposed  to 
wave  action.  It  was  during  the  period  of  rapid  submergence 
that  the  stream  valleys  of  the  present  drainage  system  were 
flooded  in  their  lower  portions  (see  Plate  III,  fig.  i,  and 
fig.  i,  page  4).  For  while  the  broad  submarine  platform 
was  being  carved  about  the  island,  whatever  subsidence 
there  may  have  been  was  not  too  rapid  for  the  cliff  cutting 
easily  to  keep  pace  with  it.  Thus  no  valley  drowning  could 
take  place,  and  no  trace  of  buried  channels  or  sunken 


70  CALIFORNIA  ACADEMY  OF  SCIENCES.        [PROC.  30  SER. 

valleys  (belonging  to  the  present  drainage  system)  is  found, 
in  general,  below  a  depth  of  250  feet. 

The  recent  tilting  of  the  island,  which  has  been  mentioned, 
appears  to  have  occurred  largely  if  not  wholly  during  the 
island's  emergence  after  its  reduction  to  a  peneplain.  For 
the  constancy  of  the  depth  of  the  more  recent  submarine 
features  clearly  shows  that  their  relative  attitude  cannot  have 
been  appreciably  altered  since  the  time  of  their  formation, 
and  therefore  that  the  tilting  must  have  preceded  this  in 
greater  part,  at  least.  To  this  recent  differential  elevation 
is  due,  in  part  at  least,  the  long,  narrow  channels  of  the 
southern  side  of  the  island,  as  contrasted  with  the  open  valleys 
on  the  north;  though  these  are  doubtless  due  in  part,  also, 
to  the  more  rapid  cliff  cutting  on  the  southern  coast. 
.  The  present  drainage  system  of  the  island  was  begun  at 
the  time  of  the  last  rise,  after  the  formation  of  the  pene- 
plain. This  peneplain  has  since  been  deeply  dissected  and 
eroded,  till  only  the  roughly  levelled  summits  of  the  ridges 
remain  to  mark  its  former  existence.  Sufficient  time  has 
elapsed  since  the  streams  began  their  work  for  the  gorge  of 
Silver  Canon  to  be  cut  down  through  more  than  1,400  feet 
of  rock,  while  in  the  same  time  the  broader  valley  back  of 
Avalon  has  been  excavated  and  its  slopes  minutely  carved. 
The  topography,  then,  is  by  no  means  young,  but  it  has  not 
passed  its  prime.  The  submergence  and  rapid  cliff  reces- 
sion tend  to  preserve  the  youthful  appearance  of  the  island, 
by  shortening  the  stream  channels,  thus  increasing  their 
grade  and  causing  the  streams  to  continue  their  sharp, 
incisive  cutting.  To  such  a  cause  is  due  the  dissection  of 
the  alluvial  fan  back  of  Avalon. 

The  isthmus  is  a  particularly  interesting  feature  of  the 
island,  for  the  mass  is  nearly  separated  at  this  point.  A 
very  slight  further  subsidence  would  be  sufficient  to  form 
two  islands.  That  the  isthmus  once  formed  a  watershed, 
which  separated  the  two  stream  valleys  to  the  north  and 
south,  there  can  be  no  doubt.  These  drowned  valleys  now 
form  the  harbors  on  either  side,  and  constitute  the  most 
pronounced  example  of  valley  drowning  on  the  island.  The 


GEOL.— VOL.  I.]        SMITH— SANTA  CATAL1NA  ISLAND.  71 

drainage  into  them  was  principally  from  the  tributary  canons. 
The  valleys  were  shallow,  with  only  a  gentle  grade  from  the 
divide  to  their  mouths,  so  that  a  comparatively  slight  sub- 
sidence has  almost  completely  drowned  them.  The  divide 
of  the  isthmus  was  at  one  time  somewhat  lower  than  at 
present,  the  pass  having  been  filled  in  to  a  certain  extent  by 
alluvial  deposits  from  the  neighboring  slopes. 

Although  the  harbors  at  the  isthmus  conform  to  the  types 
of  the  present  stream  topography,  we  cannot  suppose  that 
the  isthmus  itself  has  been  formed  wholly  by  steam  erosion 
during  the  present  topographic  cycle.  The  break  in  the 
continuity  of  the  mass,  which  is  found  at  this  point,  is  too 
sudden  and  complete  to  be  considered  as  due  to  the  forces 
of  erosion  alone,  in  view  of  the  fact  that  no  such  effect  has 
been  produced  in  any  other  portion  of  the  island.  The 
origin  of  the  isthmus  must  be  otherwise  explained.  The 
most  reasonable  hypothesis  is  that  of  a  local  sag  at  this 
point.  This  is  borne  out  by  the  sudden  change  in  the  dip 
of  the  bed  of  tuff  and  diatomaceous  earth,  as  it  approaches 
Isthmus  Cove  (shown  in  Section  A  on  the  map).  If  this  is 
due  to  a  local  depression,  that  depression  must  have  occurred 
before  the  island  had  reached  the  highest  point  in  its  last 
rise,  and  after  the  deposition  of  the  tuff  and  shale.  The 
submarine  platform  at  this  point  shows  no  apparent  depres- 
sion, so  that  any  sag  which  there  may  have  been  must  have 
taken  place  before  the  platform  was  carved. 

In  conclusion  it  must  be  said  that  the  writer's  work  upon 
the  island  was,  owing  to  limited  time,  necessarily  incom- 
plete, and  many  details  remain  for  future  investigation. 

The  writer  wishes  to  express  here  his  gratitude  to  Prof. 
Lawson  for  his  kindness  in  giving  advice  and  assistance 
throughout  the  work.  Acknowledgements  are  also  due  to 
Dr.  J.  C.  Merriam. 

Geological  Laboratory, 

University  of  California,  Oct.  ist,  1896. 


PKDC.CALACAD.SCI.  3B5ER.  GEDL  Vail 


[SMITH]  RATE  11. 


SOUTHEASTERN   EXTREMITY  OF  SANTA  CATALINA,  SHOWING  BRECCIA  NEAR  THE 

CENTER,    EXTENDING    INTO   THE    PORPHYRITE   ON   THE    LEFT. 


PRDC.CALACAD.SCI.  3TJ5EP,  GEQL..  Vail. 


[SMITH]  PLATE  III. 


FIGURE  i- BEDDING  OF  BASEMENT  ROCKS,  CHERRY  VALLEY. 


FIGURE  2-AvALON  HARBOR,  A  DROWNED  VALLEY 


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